43.字符串相乘

43.字符串相乘

涉及到的相关题目：415.字符串相加

题目描述：

解法：竖式相加：

num1 为乘数，num2为被乘数。num1与num2的每一位相乘之后再相加。

注意补 0 操作

代码：

class Solution {
    
    
    public String multiply(String num1, String num2) {
    
    
        
        StringBuilder sb = new StringBuilder();
        int len1 = num1.length(), len2 = num2.length();
        if("0".equals(num1) || "0".equals(num2)){
    
    
            return "0";
        }
        int j = len2 - 1;
        String a = charMultiStr(String.valueOf(num2.charAt(j)), num1);
        int c = 1;
        while(j > 0){
    
    
            j--;
            String b = charMultiStr(String.valueOf(num2.charAt(j)), num1);
            //补0
            int d = c;
            sb.append(b);
            while(d-- > 0){
    
    
                sb.append(0);
            }
            c++;
            a = addStrings(a, sb.toString());
            sb.delete(0, sb.length());
        }
        return a;
    }
    //一个单字符串和一个字符串相乘
    public String charMultiStr(String s, String str){
    
    
        StringBuilder sb = new StringBuilder();
        int r = 0, v= 0, a = Integer.valueOf(s);
        int i = str.length() - 1;
        while(i >= 0){
    
    
            int b = Integer.valueOf(str.charAt(i) + "");
            v = (a * b + r) % 10;
            r = (a * b + r) / 10;
            sb.append(v);
            i--;
        }
        if(r != 0){
    
    
            sb.append(r);
        }
        return sb.reverse().toString();
    }
    //两个字符串相加【题目:415.字符串相加】
    public String addStrings(String num1, String num2) {
    
    

        StringBuilder sb = new StringBuilder();

        int i = num1.length() - 1, j = num2.length() - 1;
        int r = 0, v= 0;
        while(i >= 0 || j >= 0){
    
    
            int x = i >= 0 ? num1.charAt(i) - '0' : 0;
            int y = j >= 0 ? num2.charAt(j) - '0' : 0;
            v = (r + x + y) % 10;
            r = (r + x + y) >= 10 ? 1:0;
            sb.append(v);
            i --;
            j --;
        }
        if(r!= 0){
    
    
            sb.append(r);
        }
        return sb.reverse().toString();
    }
}