Leetcode OJ: Multiply Strings

Multiply Strings

Given two numbers represented as strings, return multiplication of the numbers as a string.

Note: The numbers can be arbitrarily large and are non-negative.

大数相乘，就是模拟乘法的过程，注意好下标，别忘了最后检查高位的0就好。看代码：

class Solution {
public:
    string multiply(string num1, string num2) {
        size_t size1 = num1.size();
        size_t size2 = num2.size();
        size_t total = size1 + size2;
        vector<int> ret(size1 + size2, 0);
        // 各位相乘
        for (size_t i = 0; i < size1; ++i) {
            for (size_t j = 0; j < size2; ++j) {
                // 想明白为什么下标是i+j
                ret[i + j] += (num1[size1 - i - 1] - '0') * (num2[size2 - j - 1] - '0');
            }
        }
        // 进位一起处理
        for (size_t i = 0; i < total - 1; ++i) {
            ret[i + 1] += ret[i] / 10;
            ret[i] = ret[i] % 10;
        }
        
        size_t begin = total - 1;
        // 清理开头的0，这里是begin > 0，就是至少保留个位。
        while (begin > 0 && ret[begin] == 0)
            begin--;
        string ans(begin + 1, '0');
        for (int i = 0; i <= begin; ++i) {
            ans[i] += ret[begin-i];
        }
        return ans;
    }
};