【Leetcode】941. Valid Array of Mountains

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

---

Thought:

• First traverse from small to large to find the array subscript index of the top of the mountain
• Then traverse from big to small, if the final result is completely equal to the subscript index of the last digit of the array, returntrue
• Otherwise, returnfalse

If you think the same way, the code probably looks like this:

/*
 * @lc app=leetcode.cn id=941 lang=cpp
 *
 * [941] 有效的山脉数组
 */

// @lc code=start
class Solution {
    
    
public:
    bool validMountainArray(vector<int>& arr) {
    
    
        int len = arr.size();
        int i = 0;
        while(i < len - 1 && arr[i] < arr[i + 1])
        {
    
    
            i++;
        }

        while(i < len - 1 && arr[i] > arr[i + 1])
        {
    
    
            i++;
        }
        return i == len - 1;
    }
};
// @lc code=end

It is not difficult to find that when finding the subscript on the top of the mountain, a judgment is needed!
Thus, revised codeit was born:

/*
 * @lc app=leetcode.cn id=941 lang=cpp
 *
 * [941] 有效的山脉数组
 */

// @lc code=start
class Solution {
    
    
public:
    bool validMountainArray(vector<int>& arr) {
    
    
        int len = arr.size();
        int i = 0;
        while(i < len - 1 && arr[i] < arr[i + 1])
        {
    
    
            i++;
        }

        if(i == len - 1 || i == 0)
        {
    
    
            return false;
        }

        while(i < len - 1 && arr[i] > arr[i + 1])
        {
    
    
            i++;
        }
        return i == len - 1;
    }
};
// @lc code=end

AC：