43. Multiply Strings 大數

題目地址

把較大的數作爲被乘數，與乘數的每一位相乘，然後加到最後的結果上。
使用數組存儲最後的結果，注意相乘後的位數。

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;
            }
        }

    }
};