CF Maximum Difference Problem

#include<bits/stdc++.h>

using namespace std;

int main() {
    int n; cin >> n; // size of array
    vector<int> a(n);
    for (int i = 0; i < n; i++) cin >> a[i]; // read array
    vector<int> candidates;
    int ans = 0;
    for (int j = 0; j < n; j++) {
        // if the index is a potential answer, add it
        if (candidates.size()==0 || a[j]<a[candidates.back()]) candidates.push_back(j);
        // start the binary search, notice that candidates[candidates.size()-1] always works
        int l=0, r=candidates.size()-2, i=candidates[candidates.size()-1];
        while (l <= r) {
            int m = (l+r)/2;
            if (a[candidates[m]] <= a[j]) {
                i = candidates[m];
                r = m-1;
            } else l = m+1;
        }
        // take the maximum result of the binary search
        ans = max(ans, j-i);
    }
    cout << ans << endl;
}