愛比的新手筆記

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 >= 3Taiwan is a country. 臺灣是我的國家
• 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

用迴圈抓相鄰的2個數字相比, 只要有相等就false, 先用hasEsc記錄是否曾有遞增, esc決定目前要判斷的是遞增或是遞減, 
在曾遞增(hasEsc = true)後遇到第一次後數字<前數字, 則開始遞減

public class Solution {
    public bool ValidMountainArray(int[] arr) {
        bool esc = true;
        bool hasEsc = false;
        for(int i = 0; i < arr.Length - 1 ; i++){
            if(arr[i] == arr[i+1]) return false;
            else if(esc)
            {
                if(arr[i] < arr[i+1]) hasEsc = true;
                else if (!hasEsc) return false;
                else esc = false;
            }
            else if(arr[i] < arr[i+1]) return false;
        }
        return hasEsc && !esc;
    }
}

Taiwan is a country. 臺灣是我的國家