1064 Fixed Point

Given an array A of distinct integers sorted in ascending order, return the smallest index i that satisfies A[i] == i. Return -1 if no such i exists.

Example 1:

Input: [-10,-5,0,3,7]
Output: 3
Explanation: 
For the given array, A[0] = -10, A[1] = -5, A[2] = 0, A[3] = 3, thus the output is 3.

Example 2:

Input: [0,2,5,8,17]
Output: 0
Explanation: 
A[0] = 0, thus the output is 0.

Example 3:

Input: [-10,-5,3,4,7,9]
Output: -1
Explanation: 
There is no such i that A[i] = i, thus the output is -1.

Note:

1. 1 <= A.length < 10^4

2. -10^9 <= A[i] <= 10^9

这题经典二分，不过注意要找的是smallest，所以相等的时候不能立刻返回。要end = mid，继续向前面找。

public int fixedPoint(int[] A) {
    if (A == null || A.length == 0) {
        return -1;
    }

    int start = 0;
    int end = A.length - 1;
    while (start + 1 < end) {
        int mid = start + (end - start) / 2;
        if (A[mid] == mid) {
            end = mid;
        } else if (A[mid] < mid) {
            start = mid;
        } else {
            end = mid;
        }
    }

    if (A[start] == start) {
        return start;
    }

    if (A[end] == end) {
        return end;
    }

    return -1;
}