LeetCode415 - Add Strings 用字符串模拟整数相加

415. Add Strings

 

Given two non-negative integers num1 and num2 represented as string, return the sum of num1 and num2.

Note:

1. The length of both num1 and num2 is < 5100.
2. Both num1 and num2 contains only digits 0-9.
3. Both num1 and num2 does not contain any leading zero.
4. You must not use any built-in BigInteger library or convert the inputs to integer directly.

思路：控制num1的长度不小于num2的长度，在num1上直接做修改。

class Solution {
public:
    string addStrings(string num1, string num2) 
    {
    int len1 = num1.size(), len2 = num2.size();
    if (len2 > len1)
    {
        swap(len1, len2);
        swap(num1, num2);
    }
    int i = len1 - 1, j = len2 - 1;
    int carry = 0, a = 0, b = 0, sum = 0;
    while (i >= 0)
    {
        a = num1[i] - '0';
        if (j >= 0)
            b = num2[j] - '0';
        else
            b = 0;
        sum = a + b + carry;
        if (sum >= 10)
        {
            sum -= 10;
            carry = 1;
        }
        else
            carry = 0;
        num1[i] = sum + '0';
        --i;--j;
    }
    if (carry == 1)
        num1 = '1' + num1;
    return num1;
    }
};