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Formatted question description: https://leetcode.ca/all/665.html

665. Non-decreasing Array (Easy)

Given an array nums 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 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 can't get a non-decreasing array by modify at most one element.

 

Constraints:

  • 1 <= n <= 10 ^ 4
  • - 10 ^ 5 <= nums[i] <= 10 ^ 5

Related Topics:
Array

Solution 1.

  • class Solution {
        public boolean checkPossibility(int[] nums) {
            int length = nums.length;
            boolean flag = false;
            for (int i = 1; i < length; i++) {
                int prevNum = nums[i - 1], curNum = nums[i];
                if (prevNum > curNum) {
                    if (flag)
                        return false;
                    else {
                        if (i > 1 && i < length - 1) {
                            int prev2Num = nums[i - 2], nextNum = nums[i + 1];
                            if (curNum < prev2Num && nextNum < prevNum)
                                return false;
                        }
                        flag = true;
                    }
                }
            }
            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;
        }
    }
    
  • // OJ: https://leetcode.com/problems/non-decreasing-array/
    // Time: O(N)
    // Space: O(1)
    class Solution {
    public:
        bool checkPossibility(vector<int>& A) {
            int N = A.size(), R = N - 2;
            for (; R >= 0 && A[R] <= A[R + 1]; --R);
            for (int i = 0; i < N; ++i) {
                if ((i == 0 || i == N - 1 || A[i + 1] >= A[i - 1]) && i >= R) return true;
                if (i - 1 >= 0 && A[i] < A[i - 1]) break;
            }
            return false;
        }
    };
    
  • class Solution:
        def checkPossibility(self, nums):
            """
            :type nums: List[int]
            :rtype: bool
            """
            if len(nums) < 2:
                return True
            count = 0
            for i in range(1, len(nums)):
                if nums[i] < nums[i - 1]:
                    if count == 1:
                        return False
                    if not (
                        i + 1 == len(nums)
                        or nums[i + 1] >= nums[i - 1]
                        or i - 2 < 0
                        or nums[i - 2] < nums[i]
                    ):
                        return False
                    else:
                        count = 1
            return True
    
    ############
    
    class Solution(object):
      def checkPossibility(self, nums):
        """
        :type nums: List[int]
        :rtype: bool
        """
        flag = False
        pre = float("-inf")
        for i in range(len(nums) - 1):
          if nums[i] < pre:
            if nums[i + 1] >= nums[i - 1]:
              nums[i] = nums[i + 1]
            else:
              nums[i - 1] = nums[i]
            flag = True
            break
          pre = nums[i]
        if not flag and len(nums) > 1 and nums[-1] < nums[-2]:
          nums[-1] = nums[-2]
        pre = float("-inf")
        for num in nums:
          if num < pre:
            return False
          pre = num
        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;
    }
    
    

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