43. 字符串相乘

class Solution {
    public String multiply(String num1, String num2) {
        int n = num1.length(), m = num2.length();
        int[] A = new int[n], B = new int[m], C = new int[n + m];

        // 将num1每位数字逆序存到A数组
        for (int i = 0; i < n; i++) {
            A[i] = num1.charAt(n - i - 1) - '0';
        }
        // 将num2每位数字逆序存到B数组
        for (int i = 0; i < m; i++) {
            B[i] = num2.charAt(m - i - 1) - '0';
        }
        // 将每位相乘的结果存到C数组
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++) {
                C[i + j] += A[i] * B[j];
            }
        }
        // 处理进位
        for (int i = 0, t = 0; i < n + m; i++) {
            t += C[i];
            C[i] = t % 10;
            t /= 10;
        }

        // 去除前导0
        int k = n + m - 1;
        while (k > 0 && C[k] == 0) {
            k--;
        }
        StringBuilder res = new StringBuilder();
        // 拼接字符串
        while (k >= 0) {
            res.append((char) (C[k--] + '0'));
        }
        return res.toString();
    }
}