852.PeakIndexinaMountainArray.md

题目描述

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.

难度系数

Easy

解法一：二分查找

class Solution {
public:
    int peakIndexInMountainArray(vector<int>& A) {
        int left = 0, right = A.size() - 1;
        while (left < right) {
            int mid = (left + right) >> 1;
            if (A[mid] < A[mid + 1]) left = mid + 1;//右边一定有峰值
            else right = mid;//左边一定有峰值
        }
        return left;
    }
};

解法二：思路同解法一

class Solution:
    def peakIndexInMountainArray(self, arr: List[int]) -> int:
        left, right = 0, len(arr)
        while left != right:
            mid = (right+left) // 2
            if arr[mid - 1] < arr[mid] < arr[mid + 1]:
                left = mid + 1
            elif arr[mid - 1] > arr[mid] > arr[mid + 1]:
                right = mid
            if arr[mid - 1] < arr[mid] > arr[mid + 1]:
                return mid
        return left