0941. Valid Mountain Array (E)

Valid Mountain Array (E)

題目

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] < A[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 <= 10^4
• 0 <= arr[i] <= 10^4

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題意

判斷數組是否知足前半段單調遞增，後半段單調遞減。

思路

從前日後找遞增結束點，從後往前找遞增結束點，判斷是否重合且知足峯頂定義。

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代碼實現

Java

class Solution {
    public boolean validMountainArray(int[] arr) {
        if (arr.length < 3) {
            return false;
        }

        int left = 1, right = arr.length - 2;

        while (left < arr.length - 1 && arr[left] > arr[left - 1] && arr[left + 1] > arr[left]) {
            left++;
        }
        while (right > 1 && arr[right] > arr[right + 1] && arr[right - 1] > arr[right]) {
            right--;
        }

        return left == right && arr[left] > arr[left - 1] && arr[left] > arr[left + 1];
    }
}