Plus One

Given a non-negative number represented as an array of digits, plus one to the number.

The digits are stored such that the most significant digit is at the head of the list.

not hard

recursion

class Solution:
    # @param digits, a list of integer digits
    # @return a list of integer digits
    def plusOne(self, digits):
        if len(digits)==0:
            return []
        
        return self.recur(digits, len(digits)-1, 1)
    
    def recur(self, digits, step, carry):
        if step==-1:
            if carry==1:
                digits = [1] + digits
            return digits
        
        digits[step] = digits[step] + carry
        if digits[step] > 9:
            digits[step] = digits[step] - 10
            return self.recur(digits, step-1, 1)
        else:
            return self.recur(digits, step-1, 0);

iteration

class Solution:
    # @param digits, a list of integer digits
    # @return a list of integer digits
    def plusOne(self, digits):
        if len(digits)==0:
            return []
        
        carry = 1
        for ind in range(len(digits)-1,-1,-1):
            digits[ind] = digits[ind] + carry
            if digits[ind]>9:
                digits[ind] = digits[ind] - 10
                carry = 1
            else:
                carry = 0
        
        if carry==1:
            digits = [1] + digits
       
        return digits

Summary:

pay attention to the return when writing recursion code