From ef0ad91b89a73ad7d69f35fbbfcd66c3169e877c Mon Sep 17 00:00:00 2001
From: skrishan07 <saurabh.krishan9@gmail.com>
Date: Fri, 2 Oct 2020 19:20:54 +0530
Subject: [PATCH 1/6] Adding Leetcode monthly Challenge

---
 .../October 2020/Combination Sum.cpp          | 32 ++++++++++++++
 .../October 2020/Number of Recent Calls.cpp   | 44 +++++++++++++++++++
 .../September 2020/Find the Difference.cpp    | 24 ++++++++++
 .../September 2020/Largest Number.cpp         | 27 ++++++++++++
 4 files changed, 127 insertions(+)
 create mode 100644 Leetcode/Monthly Challenge/October 2020/Combination Sum.cpp
 create mode 100644 Leetcode/Monthly Challenge/October 2020/Number of Recent Calls.cpp
 create mode 100644 Leetcode/Monthly Challenge/September 2020/Find the Difference.cpp
 create mode 100644 Leetcode/Monthly Challenge/September 2020/Largest Number.cpp

diff --git a/Leetcode/Monthly Challenge/October 2020/Combination Sum.cpp b/Leetcode/Monthly Challenge/October 2020/Combination Sum.cpp
new file mode 100644
index 0000000..c5bea2b
--- /dev/null
+++ b/Leetcode/Monthly Challenge/October 2020/Combination Sum.cpp	
@@ -0,0 +1,32 @@
+class Solution {
+public:
+    void find( vector<int>& candidates, int target, vector<vector<int>>&ans, vector<int>v ,int start )
+    {
+        if(target==0)
+        {
+            ans.push_back(v);
+            return;
+        }
+        for(int i=start;i<candidates.size();i++)
+        {
+            if(target>=candidates[i])
+            {
+                v.push_back(candidates[i]);
+                find(candidates,target-candidates[i],ans,v,i);
+                v.pop_back();
+            }
+            else
+            {
+                break;
+            }
+        }
+    }
+    vector<vector<int>> combinationSum(vector<int>& candidates, int target) 
+    {
+        vector<vector<int>>ans;
+        sort(candidates.begin(),candidates.end());
+        vector<int>v;
+        find(candidates,target,ans,v,0);
+        return ans;
+    }
+};
\ No newline at end of file
diff --git a/Leetcode/Monthly Challenge/October 2020/Number of Recent Calls.cpp b/Leetcode/Monthly Challenge/October 2020/Number of Recent Calls.cpp
new file mode 100644
index 0000000..af58302
--- /dev/null
+++ b/Leetcode/Monthly Challenge/October 2020/Number of Recent Calls.cpp	
@@ -0,0 +1,44 @@
+class RecentCounter {
+public:
+    vector<int>v;
+    int c;
+    RecentCounter() {
+        c=0;
+    }
+    int find(vector<int>v, int t)
+    {
+        int mid=0,x=1,start=0,end=v.size()-1;
+        if(t-3000>=1)
+        {
+            x=t-3000;
+        }
+        while(start<=end)
+        {
+            mid=(start+end)/2;
+            if(v[mid]==x)
+            {
+                return mid;
+            }
+            else if(v[mid]>x)
+            {
+                end=mid-1;
+                if(end>=0 && v[end]<x)
+                {
+                    return mid;
+                }
+            }
+            else
+            {
+                start=mid+1;
+            }
+        }
+        return mid;
+    }
+    int ping(int t) {
+        c++;
+        v.push_back(t);
+        int index=find(v,t);
+        int ans=c-index;
+        return ans;
+    }
+};
\ No newline at end of file
diff --git a/Leetcode/Monthly Challenge/September 2020/Find the Difference.cpp b/Leetcode/Monthly Challenge/September 2020/Find the Difference.cpp
new file mode 100644
index 0000000..9644408
--- /dev/null
+++ b/Leetcode/Monthly Challenge/September 2020/Find the Difference.cpp	
@@ -0,0 +1,24 @@
+class Solution {
+public:
+    char findTheDifference(string s, string t) 
+    {
+        int v[26];
+        memset(v,0,sizeof(v));
+        for(int i=0;i<s.size();i++)
+        {
+            v[s[i]-97]++;
+        }
+        for(int i=0;i<t.size();i++)
+        {
+            if(v[t[i]-97])
+            {
+                v[t[i]-97]--;
+            }
+            else
+            {
+                return t[i];
+            }
+        }
+        return t[0];
+    }
+};
\ No newline at end of file
diff --git a/Leetcode/Monthly Challenge/September 2020/Largest Number.cpp b/Leetcode/Monthly Challenge/September 2020/Largest Number.cpp
new file mode 100644
index 0000000..c988288
--- /dev/null
+++ b/Leetcode/Monthly Challenge/September 2020/Largest Number.cpp	
@@ -0,0 +1,27 @@
+class Solution {
+public:
+    static bool compare(int x,int y)
+    {
+        string a=to_string(x);
+        string b=to_string(y);
+        if(a+b>b+a)
+            return true;
+        return false;
+    }
+    string largestNumber(vector<int>& nums) 
+    {
+        string x,ans="";
+        int n=nums.size();
+        if(n==0)
+            return ans;
+        sort(nums.begin(),nums.begin()+n,compare);
+        if(nums[0]==0)
+            return "0";
+        for(int i=0;i<n;i++)
+        {
+            x=to_string(nums[i]); 
+            ans+=x;
+        }
+        return ans;
+    }
+};
\ No newline at end of file

From 1ca142e942e03592cc61c8a11b13ee7b5ff91dd8 Mon Sep 17 00:00:00 2001
From: skrishan07 <saurabh.krishan9@gmail.com>
Date: Fri, 2 Oct 2020 19:39:04 +0530
Subject: [PATCH 2/6] Adding Codechef September2020 Lunchtime

---
 .../SEPT2020-LUNCHTIME/GCD operations.cpp     | 49 +++++++++++++++
 Codechef/SEPT2020-LUNCHTIME/Root the Tree.cpp | 61 +++++++++++++++++++
 Codechef/SEPT2020-LUNCHTIME/Watermelon.cpp    | 30 +++++++++
 3 files changed, 140 insertions(+)
 create mode 100644 Codechef/SEPT2020-LUNCHTIME/GCD operations.cpp
 create mode 100644 Codechef/SEPT2020-LUNCHTIME/Root the Tree.cpp
 create mode 100644 Codechef/SEPT2020-LUNCHTIME/Watermelon.cpp

diff --git a/Codechef/SEPT2020-LUNCHTIME/GCD operations.cpp b/Codechef/SEPT2020-LUNCHTIME/GCD operations.cpp
new file mode 100644
index 0000000..f835149
--- /dev/null
+++ b/Codechef/SEPT2020-LUNCHTIME/GCD operations.cpp	
@@ -0,0 +1,49 @@
+#include<bits/stdc++.h>
+#define ll long long int
+using namespace std;
+int gcd(int a, int b)
+{
+    if (b == 0)
+        return a;
+    return gcd(b, a % b);    
+}
+int main()
+{
+	ios_base::sync_with_stdio(false);
+    cin.tie(NULL);
+	int tc,n,flag,v;
+	cin>>tc;
+	while(tc--)
+	{
+		flag=1;
+		cin>>n;
+		vector<int>a(n);
+		vector<int>b(n);
+		for(int i=0;i<n;i++)
+		{
+			a[i]=i+1;
+			cin>>b[i];
+	    }
+	    for(int i=0;i<n;i++)
+	    {
+	    	if(a[i]!=b[i] && b[i]!=1)
+	    	{
+	    		v=gcd(a[i],b[i]);
+	    		if(v!=b[i])
+	    		{
+	    			flag=0;
+	    			break;
+				}
+			}
+		}
+		if(flag==0)
+		{
+			cout<<"NO"<<endl;
+		}
+		else
+		{
+			cout<<"YES"<<endl;
+		}
+	}
+	return 0;
+}
diff --git a/Codechef/SEPT2020-LUNCHTIME/Root the Tree.cpp b/Codechef/SEPT2020-LUNCHTIME/Root the Tree.cpp
new file mode 100644
index 0000000..695e533
--- /dev/null
+++ b/Codechef/SEPT2020-LUNCHTIME/Root the Tree.cpp	
@@ -0,0 +1,61 @@
+#include<bits/stdc++.h>
+#define ll long long int
+using namespace std;
+void dfs(vector<int>a[], int n, bool visited[], int &ans, bool root[])
+{
+	visited[n]=true;
+	for(int i=0;i<a[n].size();i++)
+	{
+		if(!visited[a[n][i]])
+		{
+			dfs(a,a[n][i],visited,ans,root);
+		}
+		else
+		{
+			if(root[ a[n][i] ]==true)
+			{
+				root[a[n][i]]=false;
+			}
+			else
+			{
+				ans++;
+			}
+		}
+	}
+}
+int main()
+{
+	ios_base::sync_with_stdio(false);
+    cin.tie(NULL);
+	int tc,n,x,y,ans;
+	cin>>tc;
+	while(tc--)
+	{
+		ans=0;
+		cin>>n;
+		vector<int>a[n+1];
+		bool visited[n+1];
+		bool root[n+1];
+		for(int i=1;i<n;i++)
+		{
+			visited[i]=false;
+			cin>>x>>y;
+			a[x].push_back(y);
+		}
+		for(int i=0;i<=n;i++)
+		{
+			visited[i]=false;
+			root[i]=false;
+		}
+		for(int i=1;i<=n;i++)
+		{
+			if(visited[i]==false)
+			{
+				root[i]=true;
+				dfs(a,i,visited,ans,root);
+			}
+		}
+		cout<<ans<<endl;
+	}
+	return 0;
+}
\ No newline at end of file
diff --git a/Codechef/SEPT2020-LUNCHTIME/Watermelon.cpp b/Codechef/SEPT2020-LUNCHTIME/Watermelon.cpp
new file mode 100644
index 0000000..66c75d9
--- /dev/null
+++ b/Codechef/SEPT2020-LUNCHTIME/Watermelon.cpp
@@ -0,0 +1,30 @@
+#include<bits/stdc++.h>
+#define ll long long int
+using namespace std;
+int main()
+{
+	ios_base::sync_with_stdio(false);
+    cin.tie(NULL);
+	int tc,n,sum,x;
+	cin>>tc;
+	while(tc--)
+	{
+		sum=0;
+		cin>>n;
+		vector<int>a(n);
+		for(int i=0;i<n;i++)
+		{
+			cin>>a[i];
+			sum+=a[i];
+		}
+		if(sum>=0)
+		{
+			cout<<"YES"<<endl;
+		}
+		else
+		{
+			cout<<"NO"<<endl;
+		}
+	}
+	return 0;
+}

From 82c9d911ad7f10ae28beb70f9dfd3fbc14722959 Mon Sep 17 00:00:00 2001
From: skrishan07 <saurabh.krishan9@gmail.com>
Date: Fri, 2 Oct 2020 20:08:53 +0530
Subject: [PATCH 3/6] Adding Leetcode Array Questions

---
 Leetcode/Array/Teemo Attacking.cpp | 24 +++++++++++++
 Leetcode/Array/Valid Sudoku.cpp    | 55 ++++++++++++++++++++++++++++++
 2 files changed, 79 insertions(+)
 create mode 100644 Leetcode/Array/Teemo Attacking.cpp
 create mode 100644 Leetcode/Array/Valid Sudoku.cpp

diff --git a/Leetcode/Array/Teemo Attacking.cpp b/Leetcode/Array/Teemo Attacking.cpp
new file mode 100644
index 0000000..4f192b6
--- /dev/null
+++ b/Leetcode/Array/Teemo Attacking.cpp	
@@ -0,0 +1,24 @@
+class Solution {
+public:
+    int findPoisonedDuration(vector<int>& timeSeries, int duration) 
+    {
+        int ans=0;
+        if(timeSeries.size()==0)
+        {
+            return ans;
+        }
+        ans+=duration;
+        for(int i=1;i<timeSeries.size();i++)
+        {
+            if(timeSeries[i]-timeSeries[i-1]>=duration)
+            {
+                ans+=duration;
+            }
+            else
+            {
+                ans+=timeSeries[i]-timeSeries[i-1];
+            }
+        }
+        return ans;
+    }
+};
\ No newline at end of file
diff --git a/Leetcode/Array/Valid Sudoku.cpp b/Leetcode/Array/Valid Sudoku.cpp
new file mode 100644
index 0000000..c6910c9
--- /dev/null
+++ b/Leetcode/Array/Valid Sudoku.cpp	
@@ -0,0 +1,55 @@
+class Solution {
+public:
+    int start(int n)                                        //return starting of sub-boxes
+    {
+        if(n>=0 && n<=2)
+            return 0;
+        else if(n>=3 && n<=5)
+            return 3;
+        else
+            return 6;
+    }
+    bool check(vector<vector<char>>& board, int x, int y)           //check whether board[x][y] is valid or not
+    {
+        for(int i=0;i<9;i++)                                        //Check whether column contain repetition or not
+        {
+            if(i!=x && board[i][y]==board[x][y])
+            {
+                return false;
+            }
+        }
+        for(int j=0;j<9;j++)                                        //Check whether row contain repetition or not
+        {
+            if(j!=y && board[x][j]==board[x][y])
+            {
+                return false;
+            }
+        }
+        int x1=start(x),y1=start(y);
+        for(int i=x1;i<x1+3;i++)                                   //Check whether subbox contain repetition or not
+        {
+            for(int j=y1;j<y1+3;j++)
+            {
+                if(i!=x && j!=y && board[i][j]==board[x][y])
+                {
+                    return false;
+                }
+            }
+        }
+        return true;
+    }
+    bool isValidSudoku(vector<vector<char>>& board) 
+    {
+        for(int i=0;i<9;i++)
+        {
+            for(int j=0;j<9;j++)
+            {
+                if(board[i][j]!='.' && check(board,i,j)==false)
+                {
+                    return false;
+                }
+            }
+        }
+        return true;
+    }
+};
\ No newline at end of file

From be207c603a4daea5b16d1e60961a4d1bc926cfcf Mon Sep 17 00:00:00 2001
From: skrishan07 <saurabh.krishan9@gmail.com>
Date: Sat, 3 Oct 2020 01:05:47 +0530
Subject: [PATCH 4/6] Adding Hackerrank  Questions

---
 .../BST Lowest Common Ancestor.cpp            | 23 +++++++++++++++++++
 .../Find Merge Point of Two Lists.cpp         | 19 +++++++++++++++
 2 files changed, 42 insertions(+)
 create mode 100644 Hackerrank + Geeks for Geeks + DS Courseware/BST Lowest Common Ancestor.cpp
 create mode 100644 Hackerrank + Geeks for Geeks + DS Courseware/Find Merge Point of Two Lists.cpp

diff --git a/Hackerrank + Geeks for Geeks + DS Courseware/BST Lowest Common Ancestor.cpp b/Hackerrank + Geeks for Geeks + DS Courseware/BST Lowest Common Ancestor.cpp
new file mode 100644
index 0000000..143361a
--- /dev/null
+++ b/Hackerrank + Geeks for Geeks + DS Courseware/BST Lowest Common Ancestor.cpp	
@@ -0,0 +1,23 @@
+Node *lca(Node *root, int v1,int v2) 
+{
+    Node *temp=root;
+    while(1)
+    {
+        if(v1==temp->data || v2==temp->data)
+        {
+            return temp;
+        }
+        else if(v1<temp->data && v2<temp->data)
+        {
+            temp=temp->left;
+        }
+        else if(v1>temp->data && v2>temp->data)
+        {
+            temp=temp->right;
+        }
+        else
+        {
+            return temp;
+        }
+    }
+}
\ No newline at end of file
diff --git a/Hackerrank + Geeks for Geeks + DS Courseware/Find Merge Point of Two Lists.cpp b/Hackerrank + Geeks for Geeks + DS Courseware/Find Merge Point of Two Lists.cpp
new file mode 100644
index 0000000..9f7746d
--- /dev/null
+++ b/Hackerrank + Geeks for Geeks + DS Courseware/Find Merge Point of Two Lists.cpp	
@@ -0,0 +1,19 @@
+int findMergeNode(SinglyLinkedListNode* head1, SinglyLinkedListNode* head2) 
+{
+    SinglyLinkedListNode* tmp1=head1;
+    SinglyLinkedListNode* tmp2=head2;
+    while(tmp1!=tmp2)
+    {
+        tmp1=tmp1->next;
+        tmp2=tmp2->next;
+        if(tmp1==NULL)
+        {
+            tmp1=head2;
+        }
+        if(tmp2==NULL)
+        {
+            tmp2=head1;
+        }
+    }
+    return tmp1->data;
+}
\ No newline at end of file

From 0eab1c710e2f0c8d69ef4aabd356a2f8d0d42d12 Mon Sep 17 00:00:00 2001
From: skrishan07 <saurabh.krishan9@gmail.com>
Date: Sat, 3 Oct 2020 19:31:33 +0530
Subject: [PATCH 5/6] Added Leetcode Day 3 October Challenge

---
 .../October 2020/K-diff Pairs in an Array.cpp | 33 +++++++++++++++++++
 1 file changed, 33 insertions(+)
 create mode 100644 Leetcode/Monthly Challenge/October 2020/K-diff Pairs in an Array.cpp

diff --git a/Leetcode/Monthly Challenge/October 2020/K-diff Pairs in an Array.cpp b/Leetcode/Monthly Challenge/October 2020/K-diff Pairs in an Array.cpp
new file mode 100644
index 0000000..9d2f67c
--- /dev/null
+++ b/Leetcode/Monthly Challenge/October 2020/K-diff Pairs in an Array.cpp	
@@ -0,0 +1,33 @@
+class Solution {
+public:
+    int findPairs(vector<int>& nums, int k) 
+    {
+        int ans=0;
+        unordered_map<int,int>m;
+        for(int i=0;i<nums.size();i++)
+        {                                                   
+            if(m[nums[i]])                                                  //if nums[i] is present
+            {
+                if(k==0)                                                                
+                {
+                    if(m[nums[i]]==1)
+                    {
+                        ans++;                                             //consider a=b when k=0
+                        m[nums[i]]++;
+                    }
+                }
+                continue;
+            }
+            if(m[k+nums[i]])                                               //if we consider nums[i]=a
+            {
+                ans++;
+            }
+            if(m[nums[i]-k])                                               //if we consider nums[i]=b
+            {
+                ans++;
+            }
+            m[nums[i]]=1;
+        }
+        return ans;
+    }
+};
\ No newline at end of file

From 375bfe1f64253ddf6f5931e3672398a905f5d36a Mon Sep 17 00:00:00 2001
From: skrishan07 <saurabh.krishan9@gmail.com>
Date: Fri, 1 Oct 2021 23:31:08 +0530
Subject: [PATCH 6/6] Added Longest Common Subsequence.cpp

---
 Leetcode/DP/Longest Common Subsequence.cpp | 24 ++++++++++++++++++++++
 1 file changed, 24 insertions(+)
 create mode 100644 Leetcode/DP/Longest Common Subsequence.cpp

diff --git a/Leetcode/DP/Longest Common Subsequence.cpp b/Leetcode/DP/Longest Common Subsequence.cpp
new file mode 100644
index 0000000..3029af2
--- /dev/null
+++ b/Leetcode/DP/Longest Common Subsequence.cpp	
@@ -0,0 +1,24 @@
+class Solution {
+public:
+    int longestCommonSubsequence(string text1, string text2) {
+        int n1 = text1.size();
+        int n2 = text2.size();
+        int dp[n1+1][n2+1];
+        for(int i = 0; i <= n1; i++){
+            for(int j = 0; j <= n2; j++){
+                if(i == 0 || j == 0){
+                    dp[i][j] = 0;
+                }
+                else if(text1[i-1] == text2[j-1]){
+                    dp[i][j] = 1 + dp[i-1][j-1];
+                }
+                else{
+                    dp[i][j] = max({ dp[i-1][j-1], dp[i][j-1], dp[i-1][j] });
+                }
+            }
+        }
+        return dp[n1][n2];
+    }
+};
+
+// https://leetcode.com/problems/longest-common-subsequence/
\ No newline at end of file