852. Peak Index in a Mountain Array

用binary search

如果A[mid] < A[mid+1]，说明peak在mid右侧，l = mid + 1；否则说明peak在mid右侧，r = mid

时间：O(N)，空间：O(1)

class Solution {
    public int peakIndexInMountainArray(int[] A) {
        int l = 0, r = A.length - 1;
        while(l < r) {
            int mid = l + (r - l) / 2;
            if(A[mid] < A[mid + 1])
                l = mid + 1;
            else
                r = mid;
        }
        return l;
    }
}