Index Equals Value

• Time : O(logn)
• Space : O(1)

int binarySearch(vector<int> array) {
    int l = 0, r = array.size() - 1, res = INT_MAX;
    while(l <= r) {
        int m = (l + r) / 2;
        if(array[m] == m) res = min(res, m);

        if(array[m] >= m) r = m - 1;
        else l = m + 1;
    }
    return res == INT_MAX ? -1 : res;
}

int iterationSearch(vector<int> array) {
    for(int i = 0; i < array.size(); i++)
        if(i == array[i]) return i;
    return -1;
}

int indexEqualsValue(vector<int> array) {
    return array.size() > 32 ? binarySearch(array) : iterationSearch(array);
}