大数乘法

输入两个代表整数的字符串，不使用标准库，实现大数乘法

LeetCode

//手动乘法
public String multiply(String num1, String num2) {
    if(num1.equals("0") || num2.equals("0")) return "0";
    String result = "";
    for(int i=num1.length()-1; i>=0;i--) {
        StringBuilder sb = new StringBuilder();
        for(int k=i;k<num1.length()-1;++k) sb.append('0');
        int over = 0;
        int c1 = num1.charAt(i)-'0';
        for(int j = num1.length()-1;j>=0;j--) {
            int c2 = num2.charAt(j)-'0';
            int x = c1*c2 + over;
            int y = x % 10;
            sb.insert(0, y);
            over = x / 10;
        }

        if (over>0) {
            sb.insert(0, over);
        }

        String tmp = sb.toString();
        result = add(result, tmp);
    }

    return result;
}

//字符串加法
public String add(String s1, String s2) {
    int min = Math.min(s1.length(), s2.length());
    int over = 0;
    StringBuilder sb = new StringBuilder();
    for(int i = 0; i<min;++i) {
        int end1 = s1.length() -1 - i;
        int end2 = s2.length() -1 - i;

        int c1 = s1.charAt(end1) - '0';
        int c2 = s2.charAt(end2) - '0';

        int x = c1 + c2 + over;
        sb.insert(0, x%10);
        over = x/10;
    }

    for(int i=s1.length()-1-min;i>=0;--i) {
        int c = s1.charAt(i) - '0';
        int x = c + over;
        sb.insert(0, x%10);
        over = x/10;
    }

    for(int i=s2.length()-1-min;i>=0;--i) {
        int c = s2.charAt(i) - '0';
        int x = c + over;
        sb.insert(0, x%10);
        over = x/10;
    }

    if (over>0) {
        sb.insert(0, over);
    }

    return sb.toString();
}