852. Peak Index in a Mountain Array

Question:

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:

  1. 3 <= A.length <= 10000
  2. 0 <= A[i] <= 10^6
  3. A is a mountain, as defined above.

Solution:

  • Method 1: Binary search 
      class Solution {
      public int peakIndexInMountainArray(int[] A) {
          int low = 0, high = A.length - 1;
          while(low < high){
              int mid = low + (high - low) / 2;
              if(A[mid] < A[mid + 1]) low = mid + 1;
              else high = mid;
          }
          return low;
      }
  }