1064. Fixed Point

Given an array of distinct integers arr, where arr is sorted in ascending order, return the smallest index i that satisfies arr[i] == i. If there is no such index, return -1.

class Solution {
public:
    int fixedPoint(vector<int>& arr) {
        int index = lower_bound(arr.begin(), arr.end(), 0) - arr.begin();
        
        while(index < arr.size()) {
            if(index == arr[index]) return index;
            else if(index > arr[index]) index++;
            else index = arr[index] + 1;
        }
        return -1;
    }
};