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.

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实现字符串加法，按照小学老师教的，从低到高一位一位加，注意保留进位。这题仔细一点，然后调试两次就差不多能AC了。 代码中需要注意运算符优先级，加法优先级高于三目运算符，所以三目运算符要加括号。 完整代码如下： [cpp] class Solution { public: string addStrings(string num1, string num2) { string ans = ""; int i = num1.size() – 1, j = num2.size() – 1; bool carry = false; while (i >= 0 || j >= 0) { int sum = 0; if (i < 0)sum = (num2[j] – ‘0’) + (carry ? 1 : 0); else if (j < 0)sum = (num1[i] – ‘0’) + (carry ? 1 : 0); else sum = (num1[i] – ‘0’) + (num2[j] – ‘0’) + (carry ? 1 : 0); //注意运算符优先级，要加括号 if (sum >= 10) { carry = true; sum -= 10; } else { carry = false; } char c = ‘0’ + sum; ans = c + ans; i–; j–; } if (carry)return ‘1’ + ans; else return ans; } }; [/cpp] 本代码提交AC，用时12MS。]]>