【leetcode】941. Valid Mountain Array

Given an array of integers arr, return true if and only if it is a valid mountain array.

Recall that arr is a mountain array if and only if:

• arr.length >= 3
• There exists some i with 0 < i < arr.length - 1 such that:
  • arr[0] < arr[1] < ... < arr[i - 1] < arr[i]
  • arr[i] > arr[i + 1] > ... > arr[arr.length - 1]

Example 1:

Input: arr = [2,1]
Output: false

Example 2:

Input: arr = [3,5,5]
Output: false

Example 3:

Input: arr = [0,3,2,1]
Output: true

Constraints:

• 1 <= arr.length <= 104
• 0 <= arr[i] <= 104

class Solution {
public:
    bool validMountainArray(vector<int>& arr) {
        
        int flag=0; // 梯度正负变化flag
        if(arr.size()<3|| (arr[1]-arr[0])<=0) return false;
        int le=(arr[1]-arr[0]);
        for(int i=2;i<arr.size();++i){
            le*=(arr[i]-arr[i-1]);
            
            if(le<0) flag++;// 可能找到了拐点
            else if(le==0) flag=2;
            
            if(flag>1) break;
            le=arr[i]-arr[i-1];
        }
        return flag==1; // mountain array 只能有一个拐点
    }
};