Question

665. Non-decreasing Array

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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

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

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;
  }
}

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;
  }
};

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
}

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;
}