105.从前序与中序遍历序列构造二叉树.cpp

/*
 * @lc app=leetcode.cn id=105 lang=cpp
 *
 * [105] 从前序与中序遍历序列构造二叉树
 */

// @lc code=start
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    unordered_map<int, int> pos;
    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        int n = inorder.size(), m = preorder.size();
        for (int i = 0; i < n; i++) pos[inorder[i]] = i;
        return dfs(preorder, inorder, 0, m - 1, 0, n - 1);
    }
    TreeNode* dfs(vector<int>& preorder, vector<int>& inorder, int pl, int pr, int il, int ir) {
        if (pl > pr) return nullptr;
        auto root = new TreeNode(preorder[pl]);
        int k = pos[root->val];
        root->left = dfs(preorder, inorder, pl + 1, pl + 1 + k - 1 - il, il, k - 1);
        root->right = dfs(preorder, inorder, pl + 1 + k - 1 - il + 1, pr, k + 1, ir);
        return root;
    }
};
// @lc code=end