Leetcode skimming ---852

subject ： Arrays that match the following properties arr be called Mountains array ：

•  arr.length >= 3
• There is i（0 < i < arr.length - 1） bring ：
• arr[0] < arr[1] < ... arr[i-1] < arr[i]
• arr[i] > arr[i+1] > ... > arr[arr.length - 1]

Here's an array of mountains made up of integers arr , Return anything that satisfies arr[0] < arr[1] < ... arr[i - 1] < arr[i] > arr[i + 1] > ... > arr[arr.length - 1] The subscript i .

Input ：arr = [0,2,1,0]

Output ：1

Method 1 ： enumeration

class Solution {
public:
    int peakIndexInMountainArray(vector<int>& arr) {
        int n = arr.size();
        int ans = -1;
        for (int i = 1; i < n - 1; ++i) {
            if (arr[i] > arr[i + 1]) {
                ans = i;
                break;
            }
        }
        return ans;
    }
};

Complexity analysis

Time complexity ：O(n)

Spatial complexity ：O(1)

Method 2 ： Two points search

class Solution {
public:
    int peakIndexInMountainArray(vector<int>& arr) {
        int n = arr.size();
        int left = 1, right = n - 2, ans = 0;
        while (left <= right) {
            int mid = (left + right) / 2;
            if (arr[mid] > arr[mid + 1]) {
                ans = mid;
                right = mid - 1;
            } else {
                left = mid + 1;
            }
        }
        return ans;
    }
};

Complexity analysis

Time complexity ：O(logn)

Spatial complexity ：O(1)