66-Plus One

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题意

Given a non-empty array of decimal digits representing a non-negative integer, increment one to the integer. The digits are stored such that the most significant digit is at the head of the list, and each element in the array contains a single digit. You may assume the integer does not contain any leading zero, except the number 0 itself.

Example 1:
Input: digits = [1,2,3]
Output: [1,2,4]
Explanation: The array represents the integer 123.

Example 2:
Input: digits = [4,3,2,1]
Output: [4,3,2,2]
Explanation: The array represents the integer 4321.

Example 3:
Input: digits = [0]
Output: [1]

思路

• 数学题：加法进位
• 倒序遍历：讨论当前数位是否<9

class Solution(object):
    def plusOne(self, digits):
        """
        :type digits: List[int]
        :rtype: List[int]
        """
        if not digits: return []
        
        # 数位倒序遍历
        # 遇到<9的数位 + 1
        # 遇到9的数位 -> 0,前一位加1 -> 所有数位都为0，最前面加1 10000
        for i in range(len(digits)-1, -1, -1):
            if digits[i] < 9: 
                digits[i] += 1
                return digits
            else:
                digits[i] = 0    
        
        digits.insert(0,1)
        return digits

分析:

• Time: O(n) 遍历数字位数
• Space: O(1)