findLongestSequence.cc

#include <bits/stdc++.h>

using namespace std;

int findSequence(vector<int> v) {
    unordered_set<int> hashSet;

    for(auto i = v.begin(); i != v.end(); i++) {
        hashSet.insert(*i);
    }

    int length = 0;
    int max = 0;

    for(auto i = v.begin(); i != v.end(); i++) {
        if (hashSet.find(*i - 1) != hashSet.end()) {
            continue;
        }
        int start = *i;
        while (hashSet.find(start++) != hashSet.end()) {
            length++;
        }
        if (length > max) {
            max = length;
            length = 0;
        }
    }
    return max;
}

int main() {
    vector<int> v = {1,4,5,6,3};
    cout << findSequence(v) << endl;
    return 0;
}