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852. Peak Index in a Mountain Array

size prop ini length inpu mon defined index pro

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.
    因為山峰的數字肯定是大於左邊的數字跟右邊的數字,所以用二分查找左右逼近找速度比較快。

     1 int peakIndexInMountainArray(int* A, int ASize) {
     2     int low=0,high=ASize,mid;
    
    3 while(low<high){ 4 mid=low+(high-low)/2; 5 if(A[mid]>A[mid+1]) high=mid; 6 else low=mid+1; 7 8 } 9 return low; 10 }

852. Peak Index in a Mountain Array