Leetcode(43) Multiply Strings

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Description

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:

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

Example 2:

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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.

解法

其实就是大数相乘的算法，这里需要注意的是在每次做乘法运算时，不考虑进位问题，直接把结果存入结果数组的对应位上，最后统一处理进位问题。注意，计算从数组的尾部开始。

具体代码如下：

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class Solution {
    public String multiply(String num1, String num2) {
        int l1 = num1.length();
        int l2 = num2.length();
        int[] result = new int[l1 + l2];
        int k = l1 + l2 - 1;
        for (int j = l2 - 1; j >= 0; j--) {
            int kk = k;
            for (int i = l1 - 1; i >= 0; i--) {
                result[kk] += (num2.charAt(j) - '0') * (num1.charAt(i) - '0');
                kk--;
            }
            k--;
        }
        for (k = l1 + l2 - 1; k >= 0; k--) {
            int j = result[k] / 10;
            result[k] = result[k] % 10;
            if (k >= 1) {
                result[k - 1] += j;
            }
        }
        StringBuffer s = new StringBuffer();
        int i = 0;
        for (; i < result.length && result[i] == 0; i++) ;
        for (; i < result.length; i++) {
            s.append(result[i]);
        }
        if (s.length() == 0) {
            return "0";
        }
        String res = new String(s);
        return res;
    }
}