Binary search O(logn)
due to its property of ascending order
class Solution {
public int fixedPoint(int[] arr) {
int n = arr.length;
int l = 0, r= n-1;
while (l+1<r){
int mid = l+ (r-l)/2;
if (arr[mid]>=mid) r = mid;
else l = mid+1;
}
if (arr[l] == l) return l;
if (arr[r] ==r) return r;
return -1;
}
}