Find Peak Element

Description

There is an integer array which has the following features:

The numbers in adjacent positions are different.

A[0] < A[1] && A[A.length - 2] > A[A.length - 1].

We define a position P is a peek if:

A[P] > A[P-1] && A[P] > A[P+1]

Find a peak element in this array. Return the index of the peak.

**Notice

**

The array may contains multiple peeks, find any of them.

Example

Given[1, 2, 1, 3, 4, 5, 7, 6]

Return index1(which is number 2) or6(which is number 7)

Challenge

Time complexity O(logN)

Related problems

Maximum Number in Mountain Sequence

Find Peak Element II

Implementation

Link: http://lintcode.com/en/problem/strstr/

class Solution {
public:
    /**
     * @param A: An integers array.
     * @return: return any of peek positions.
     */
    int findPeak(vector<int> A) {
        // write your code here
        if(A.size() <= 2) return -1;

        int left = 1;
        int right = A.size() - 1;

        while(left + 1 < right){
            int mid = left + (right - left)/2;

            if(A[mid] < A[mid - 1]){
                right = mid;
            }else {
                left = mid;
            }
        }

        return A[left] < A[right] ? right : left;

    }
};