Peak Index in a Mountain Array

LintCode Q 852 - Peak Index in a Mountain Array

Let’s call an array A a mountain if the following properties hold:

• A.length >= 3
• There exists some 0 < i < A.length - 1 such that A[0] < A[1] < ... A[i-1] < A[i] > A[i+1] > ... > A[A.length - 1]
• Given an array that is definitely a mountain, return any i such that A[0] < A[1] < ... A[i-1] < A[i] > A[i+1] > ... > A[A.length - 1].

Example 1: Input: [0,1,0] ; Output: 1
Example 2: Input: [0,2,1,0] ; Output: 1

Note:

• 3 <= A.length <= 10000
• 0 <= A[i] <= 10^6
• A is a mountain, as defined above.

Solution

public int peakIndexInMountainArray(int[] A) {
  int left = 0, right = A.length;
  while (left < right) {
    int mid = (left + right) / 2;
    if (A[mid] < A[mid + 1])
      left = mid + 1;
    else
      right = mid;
  }
  
  return left;
}