Non-decreasing Array

https://leetcode.com/problems/non-decreasing-array/

 

class Solution:
    def checkPossibility(self, nums: List[int]) -> bool:
        changed = False

        for i in range(len(nums) - 1):
            if nums[i] <= nums[i+1]:
                continue
            if changed:
                return False

            # We want to decrease left element , if possible

            if i == 0 or  nums[i+1] >= nums[i-1]:
                nums[i] = nums[i+1]
            else:
                nums[i+1] = nums[i]
            changed = True
        return True

when considering pair, we can either adjust left element to be same as right one or adjust right element to be same as left element.

we chose to do left adjustment. only if i-1 is less than i+1  , otherwise it would ruin previous non decreasing order