43. Multiply Strings 大数

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题目地址

把较大的数作为被乘数，与乘数的每一位相乘，然后加到最后的结果上。
使用数组存储最后的结果，注意相乘后的位数。

class Solution {public:    string multiply(string num1, string num2) {        string ans;        const int maxn = num1.size() + num2.size() + 5;        int a[maxn] = {0};        reverse(num1.begin(), num1.end());        reverse(num2.begin(), num2.end());        if (num2.size() > num1.size()) num1.swap(num2);        for (int i = 0; i < num2.size(); ++i) {            add(maxn, a, num1, num2, i);        }        int index = maxn - 1;        while (index >= 0 && a[index] == 0) index--;        for (int i = index; i >= 0; --i) {            ans += a[i] + '0';        }        if (ans.size() == 0) ans += '0';        return ans;    }    void add(const int maxn, int a[], const string &num1, const string &num2, int n) {        int carry = 0, n1 = num2[n] - '0';        int b[num1.size() + 5] = {0};        for (int i = 0; i < num1.size(); ++i) {            int n2 = num1[i] - '0';            int now = (n1 * n2 + carry) % 10;            carry = (n1 * n2 + carry) / 10;            b[i] = now;        }        if (carry) b[num1.size()] = carry;        carry = 0;        for (int i = 0; i + n < maxn; ++i) {            if (i < num1.size() + 1) {                int now = (a[i + n] + b[i] + carry) % 10;                carry = (a[i + n] + b[i] + carry) / 10;                a[i + n] = now;            } else {                int now = (a[i + n] + carry) % 10;                carry = (a[i] + carry) / 10;                a[i + n] = now;            }        }    }};

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