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665. Non-decreasing Array

Description

Given an array nums with n integers, your task is to check if it could become non-decreasing by modifying at most one element.

We define an array is non-decreasing if nums[i] <= nums[i + 1] holds for every i (0-based) such that (0 <= i <= n - 2).

 

Example 1:

Input: nums = [4,2,3]
Output: true
Explanation: You could modify the first 4 to 1 to get a non-decreasing array.

Example 2:

Input: nums = [4,2,1]
Output: false
Explanation: You cannot get a non-decreasing array by modifying at most one element.

 

Constraints:

  • n == nums.length
  • 1 <= n <= 104
  • -105 <= nums[i] <= 105

Solutions

Solution 1

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class Solution:
    def checkPossibility(self, nums: List[int]) -> bool:
        def is_sorted(nums: List[int]) -> bool:
            return all(a <= b for a, b in pairwise(nums))

        n = len(nums)
        for i in range(n - 1):
            a, b = nums[i], nums[i + 1]
            if a > b:
                nums[i] = b
                if is_sorted(nums):
                    return True
                nums[i] = nums[i + 1] = a
                return is_sorted(nums)
        return True

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class Solution {
    public boolean checkPossibility(int[] nums) {
        for (int i = 0; i < nums.length - 1; ++i) {
            int a = nums[i], b = nums[i + 1];
            if (a > b) {
                nums[i] = b;
                if (isSorted(nums)) {
                    return true;
                }
                nums[i] = a;
                nums[i + 1] = a;
                return isSorted(nums);
            }
        }
        return true;
    }

    private boolean isSorted(int[] nums) {
        for (int i = 0; i < nums.length - 1; ++i) {
            if (nums[i] > nums[i + 1]) {
                return false;
            }
        }
        return true;
    }
}

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class Solution {
public:
    bool checkPossibility(vector<int>& nums) {
        int n = nums.size();
        for (int i = 0; i < n - 1; ++i) {
            int a = nums[i], b = nums[i + 1];
            if (a > b) {
                nums[i] = b;
                if (is_sorted(nums.begin(), nums.end())) {
                    return true;
                }
                nums[i] = a;
                nums[i + 1] = a;
                return is_sorted(nums.begin(), nums.end());
            }
        }
        return true;
    }
};

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func checkPossibility(nums []int) bool {
    isSorted := func(nums []int) bool {
        for i, b := range nums[1:] {
            if nums[i] > b {
                return false
            }
        }
        return true
    }
    for i := 0; i < len(nums)-1; i++ {
        a, b := nums[i], nums[i+1]
        if a > b {
            nums[i] = b
            if isSorted(nums) {
                return true
            }
            nums[i] = a
            nums[i+1] = a
            return isSorted(nums)
        }
    }
    return true
}

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function checkPossibility(nums: number[]): boolean {
    const isSorted = (nums: number[]) => {
        for (let i = 0; i < nums.length - 1; ++i) {
            if (nums[i] > nums[i + 1]) {
                return false;
            }
        }
        return true;
    };
    for (let i = 0; i < nums.length - 1; ++i) {
        const a = nums[i],
            b = nums[i + 1];
        if (a > b) {
            nums[i] = b;
            if (isSorted(nums)) {
                return true;
            }
            nums[i] = a;
            nums[i + 1] = a;
            return isSorted(nums);
        }
    }
    return true;
}

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