leetcode 665. Non-decreasing Array

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).

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.

题目大意， 判断原序列在至多修改一个位置后，能不能够成一个上升序列。

我们只需要求原序列的最长上升序列的长度就可以了。如果n - 最长上升序列的长度 <= 1 那么就返回true。

class Solution {
public:
    bool checkPossibility(vector<int>& nums) {
        int n = nums.size();
        if (n == 1) return true;
        vector<int> d;
        d.emplace_back(nums[0]);
        for (int i = 1; i < nums.size(); ++i) {
            if (nums[i] >= d.back()) d.emplace_back(nums[i]);
            else {
                int pos = lower_bound(d.begin(), d.end(), nums[i]) - d.begin();
                if (pos >= d.size()) d.emplace_back(nums[i]);
                else d[pos] = nums[i];
            }
        }
        //cout << d.size() << endl;
        if (n - d.size() <= 1) return true;
        return false;
    }
};