[LeetCode] 43. Multiply Strings 字符串相乘

Given two non-negative integers num1 and num2 represented as strings, return the product of num1 and num2, also represented as a string.

Example 1:

Input: num1 = "2", num2 = "3"
Output: "6"

Example 2:

Input: num1 = "123", num2 = "456"
Output: "56088"

Note:

1. The length of both num1 and num2 is < 110.
2. Both num1 and num2 contain only digits 0-9.
3. Both num1 and num2 do not contain any leading zero, except the number 0 itself.
4. You must not use any built-in BigInteger library or convert the inputs to integer directly.

求两个字符串数字的相乘，输入的两个数和返回的数都是以字符串格式储存，这样做的原因可能是计算超大数相乘不受数值范围的约束。题目也要求不能用内置长整数计算。

解法：用数组来记录每一位的计算结果，最后在转成字符串就可以了。

Java:

?

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

public String multiply(String num1, String num2) {     int m = num1.length(), n = num2.length();     int[] pos = new int[m + n];          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');             int p1 = i + j, p2 = i + j + 1;             int sum = mul + pos[p2];               pos[p1] += sum / 10;             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(); }

Python:

?

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

class Solution(object):     def multiply(self, num1, num2):         """         :type num1: str         :type num2: str         :rtype: str         """         num1, num2 = num1[::-1], num2[::-1]         res = [0] * (len(num1) + len(num2))         for i in xrange(len(num1)):             for j in xrange(len(num2)):                 res[i + j] += int(num1[i]) * int(num2[j])                 res[i + j + 1] += res[i + j] / 10                 res[i + j] %= 10           # Skip leading 0s.         i = len(res) - 1         while i > 0 and res[i] == 0:             i -= 1           return ''.join(map(str, res[i::-1]))

C++:

?

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

class Solution { public:     string multiply(string num1, string num2) {         string res;         int n1 = num1.size(), n2 = num2.size();         int k = n1 + n2 - 2, carry = 0;         vector<int> v(n1 + n2, 0);         for (int i = 0; i < n1; ++i) {             for (int j = 0; j < n2; ++j) {                 v[k - i - j] += (num1[i] - '0') * (num2[j] - '0');             }         }         for (int i = 0; i < n1 + n2; ++i) {             v[i] += carry;             carry = v[i] / 10;             v[i] %= 10;         }         int i = n1 + n2 - 1;         while (v[i] == 0) --i;         if (i < 0) return "0";         while (i >= 0) res.push_back(v[i--] + '0');         return res;     } };

　　

 

All LeetCode Questions List 题目汇总