0941. Valid Mountain Array (E)

题目

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

题意

判断数组是否满足前半段单调递增，后半段单调递减。

思路

从前往后找递增结束点，从后往前找递增结束点，判断是否重合且满足峰顶定义。

代码实现

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];
    }
}