LeetCode 852. Peak Index in a Mountain Array

Let's call an array arr a mountain if the following properties hold:

• arr.length >= 3
• There exists some i with 0 < i < arr.length - 1 such that:
  • arr[0] < arr[1] < ... arr[i-1] < arr[i]
  • arr[i] > arr[i+1] > ... > arr[arr.length - 1]

Given an integer array arr that is guaranteed to be a mountain, return any i such that arr[0] < arr[1] < ... arr[i - 1] < arr[i] > arr[i + 1] > ... > arr[arr.length - 1].

 

Example 1:

Input: arr = [0,1,0]
Output: 1

Example 2:

Input: arr = [0,2,1,0]
Output: 1

Example 3:

Input: arr = [0,10,5,2]
Output: 1

 

Constraints:

• 3 <= arr.length <= 104
• 0 <= arr[i] <= 106
• arr is guaranteed to be a mountain array.

Solution:

class Solution {
    public int peakIndexInMountainArray(int[] A) {
        int lo = 0, hi = A.length - 1;
        while (lo < hi) {
            int mid = lo + (hi - lo) / 2;
            if (A[mid] < A[mid + 1]) { 
                // peak index is after mid.
                lo = mid + 1;
            }else if (A[mid -1] > A[mid]) { 
                // peak index is before mid.
                hi = mid;
            }else { // peak index is mid.
                return mid;
            }
        }
        return -1; // no peak.
    }
}