43. Multiply Strings

43 Multiply Strings

关键词，进位。

public class Solution {
    public String multiply(String num1, String num2) {
        int m = num1.length(), n = num2.length();
        int[] pos = new int[m + n]; // 0是最高位
       
        for(int i = m - 1; i >= 0; i--) {
            for(int j = n - 1; j >= 0; j--) {
                int mul = (num1.charAt(i) - '0') * (num2.charAt(j) - '0'); 
                // p1代表进位， p2表示本位。
                int p1 = i + j, p2 = i + j + 1;
                // sum 就是本位的乘积加上本位已有的值。
                int sum = mul + pos[p2];
                // 进位就是除以10的余数
                pos[p1] += sum / 10;
                // 本位就是剩下的个位数。
                pos[p2] = (sum) % 10;
            }
        }  
        
        StringBuilder sb = new StringBuilder();
        for(int p : pos) if(!(sb.length() == 0 && p == 0)) sb.append(p);
        return sb.length() == 0 ? "0" : sb.toString();
    }
}

67 Add Binary

public class Solution {
    public String addBinary(String a, String b) {
        StringBuilder sb = new StringBuilder();
        
        char[] cha = a.toCharArray();
        char[] chb = b.toCharArray();
        int i = cha.length-1, j = chb.length -1, carry = 0;
        
        while(i >=0 || j>=0) {
            int vala = i >= 0 ? cha[i--] - '0' : 0;
            int valb = j >= 0 ? chb[j--] - '0' : 0;
            int cur = vala+ valb + carry;
            sb.append(cur%2);
            carry = cur/2;
        }
        if(carry == 1) sb.append(1);
        return sb.reverse().toString();
    }
}

415 Add Strings

public class Solution {
    public String addStrings(String num1, String num2) {
        StringBuilder sb = new StringBuilder();
        
        for(int i = num1.length() -1, j = num2.length()-1, carry = 0; i >= 0 || j >= 0 || carry == 1; i--, j--){
            int x = i < 0 ? 0 : num1.charAt(i) - '0';
            int y = j < 0 ? 0 : num2.charAt(j) - '0';
            sb.append((x+y+carry)%10);
            carry = (x+y+carry)/10;
        }
        
        return sb.reverse().toString();
    }
}