leetcode 665. Non-decreasing Array

1.题目

Given an array with n integers, your task is to check if it could become non-decreasing by modifying at most 1 element.
We define an array is non-decreasing if array[i] <= array[i + 1] holds for every i (1 <= i < n).

给出一个数组，判断是否能够只改变一个元素使这个数组变为非递减数组。 array[i] <= array[i + 1]，i属于[1，n)
Example 1:
Input: [4,2,3]
Output: True
Explanation: You could modify the first 4 to 1 to get a non-decreasing array.
Example 2:
Input: [4,2,1]
Output: False
Explanation: You can’t get a non-decreasing array by modify at most one element.
Note: The n belongs to [1, 10,000].

2.分析

一次遍历数组，用计数器count记录出现不符合a[i] <= a[i + 1]的次数

出现第一次时，如果此时位置为i，即 a[i-1] > a[i],此时需要改变一个元素的值，使其符合非递减的规则。
判断:
如果i==1 或者 a[i-2]<=a[i]，则令 a[i-1]=a[i] 那么就是 a[i-2]

3.代码

class Solution {public:    bool checkPossibility(vector<int>& nums) {        int count = 0;        for (int i = 1; i < nums.size()&&count<2; i++) {            if (nums[i] < nums[i - 1]) {                if (i == 1 || nums[i - 2] <= nums[i])                    nums[i - 1] = nums[i];                else                    nums[i] = nums[i - 1];                count += 1;            }        }        return count < 2;    }};