Correct SparseTable.cpp vi allocations

[JLeow00]

P2 and L2 are not allocated enough space because the loop in the next line would assign P2[L2_n] and L2[(1<<L2_n)] which would be out of range

[BrandonTang89]

Yeah, apart from that, we can go one step further to allocate just enough memory in SpT so as to not waste space!

class SparseTable {  // OOP style
   private:
    vi A, P2, L2;
    vector<vi> SpT;  // the Sparse Table
   public:
    SparseTable() {}  // default constructor

    SparseTable(vi &initialA) {  // pre-processing routine
        A = initialA;
        int n = (int)A.size();
        int L2_n = (int)log2(n) + 1;
        P2.assign(L2_n + 1, 0);
        L2.assign((1 << L2_n) + 1, 0);
        for (int i = 0; i <= L2_n; ++i) {
            P2[i] = (1 << i);  // to speed up 2^i
            L2[(1 << i)] = i;  // to speed up log_2(i)
        }
        for (int i = 2; i < P2[L2_n]; ++i)
            if (L2[i] == 0) L2[i] = L2[i - 1];  // to fill in the blanks

        // the initialization phase
        SpT = vector<vi>(L2[n] + 1, vi());
        SpT[0] = vi(n, 0);
        for (int j = 0; j < n; ++j) SpT[0][j] = j;  // RMQ of sub array [j..j]

        // the two nested loops below have overall time complexity = O(n log n)
        for (int i = 1; P2[i] <= n; ++i) {             // for all i s.t. 2^i <= n
            SpT[i] = vi(n + 1 - P2[i]);                // initialize SpT[i]
            for (int j = 0; j + P2[i] - 1 < n; ++j) {  // for all valid j
                int x = SpT[i - 1][j];                 // [j..j+2^(i-1)-1]
                int y = SpT[i - 1][j + P2[i - 1]];     // [j+2^(i-1)..j+2^i-1]
                SpT[i][j] = A[x] <= A[y] ? x : y;
            }
        }
    }

    int RMQ(int i, int j) {
        int k = L2[j - i + 1];          // 2^k <= (j-i+1)
        int x = SpT[k][i];              // covers [i..i+2^k-1]
        int y = SpT[k][j - P2[k] + 1];  // covers [j-2^k+1..j]
        return A[x] <= A[y] ? x : y;
    }
};

[fbrunodr]

#107 Found the same problem. This seems like a serious error, as this bug may lead to a really hard to find runtime error. Sadly It seems like @stevenhalim is not updating the code anymore.

[stevenhalim]

Thanks for the bug fix. I was so busy recently and didn't check this repo that often.