622C - Not Equal on a Segment - CodeForces Solution

#include <bits/stdc++.h>

using namespace std;

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    int n, m;
    cin >> n >> m;
    vector<int> a(n);
    for (int i = 0; i < n; ++i ) cin >> a[i];
    vector<int> prec(n, -1);
    for (int i = 1; i < n; ++i){
        if (a[i - 1] != a[i]) prec[i] = i - 1;
        else prec[i] = prec[i - 1];
    }
    for (int i = 0; i < m; ++i){
        int l, r, x;
        cin >> l >> r >> x;
        if (a[r - 1] != x) cout << r;
        else if (prec[r - 1] >= l - 1) cout << prec[r - 1] + 1;
        else cout << -1;
        cout << "\n";
    }
    return 0;
}