[leetcode] Multiply Strings

Given two numbers represented as strings, return multiplication of the numbers as a string.

Note: The numbers can be arbitrarily large and are non-negative.

https://oj.leetcode.com/problems/multiply-strings/

思路：大数乘法，模拟笔算过程即可。

注意的细节：

1. 前导0去掉；
2. 结果为0；

代码：

import java.util.Arrays;

public class Solution {
	public String multiply(String num1, String num2) {
		if (num1 == null || num2 == null || num1.length() == 0
				|| num2.length() == 0)
			return null;
		int n1 = num1.length();
		int n2 = num2.length();

		char[] str1 = num1.toCharArray();
		char[] str2 = num2.toCharArray();
		char[] prod = new char[n1 + n2];

		Arrays.fill(prod, '0');

		int i, j;
		for (i = 0; i < n1; i++)
			for (j = 0; j < n2; j++) {
				prod[i + j] = (char) ((prod[i + j] - '0')
						+ (str1[n1 - 1 - i] - '0') * (str2[n2 - 1 - j] - '0') + '0');
			}


		int c = 0;
		for (i = 0; i < n1 + n2; i++) {
			prod[i] = (char) ((prod[i] - '0') + c + '0');
			if (prod[i] - '0' + c > 9) {
				c = (prod[i] - '0') / 10;
				prod[i] = (char) ((prod[i] - '0') % 10 + '0');
			} else
				c = 0;
		}


		StringBuilder sb = new StringBuilder();

		i = n1 + n2 - 1;
		while (i >= 0 && prod[i] == '0')
			i--;
		if (i == -1) {
			sb.append("0");
			return sb.toString();
		}

		for (; i >= 0; i--) {
			sb.append(prod[i]);
		}

		return sb.toString();
	}

	public static void main(String[] args) {
		System.out.println(new Solution().multiply("123", "12345"));
		System.out.println(new Solution().multiply("0", "123"));
		
	}
}

第二遍记录：

　　prod用int数组更方便些

　　prod[i+j] += ，加号不要忘记了，是多个乘积累加。

　　注意字符串顺序与索引是反的。

public class Solution {
    public String multiply(String num1, String num2) {
        if (num1 == null || num2 == null || num1.length() == 0 || num2.length() == 0)
            return null;
        int n1 = num1.length();
        int n2 = num2.length();
        int[] prod = new int[n1 + n2];

        for (int i = 0; i < n1; i++) {
            for (int j = 0; j < n2; j++) {
                prod[i + j] += (num1.charAt(n1 - 1 - i) - '0') * (num2.charAt(n2 - 1 - j) - '0');
            }
        }

        int c = 0;
        for (int i = 0; i < n1 + n2; i++) {
            int curVal = prod[i] + c;
            if (curVal > 9) {
                c = curVal / 10;
                prod[i] = curVal % 10;
            } else {
                prod[i] = curVal;
                c = 0;
            }
        }
        StringBuilder sb = new StringBuilder();
        int idx = n1 + n2 - 1;
        while (idx >= 0 && prod[idx] == 0)
            idx--;
        if (idx == -1) {
            sb.append(0);
            return sb.toString();
        }

        while (idx >= 0)
            sb.append(prod[idx--]);

        return sb.toString();

    }

}

参考：

http://www.cnblogs.com/TenosDoIt/p/3735309.html

http://leetcodenotes.wordpress.com/2013/10/20/leetcode-multiply-strings-%E5%A4%A7%E6%95%B4%E6%95%B0%E7%9A%84%E5%AD%97%E7%AC%A6%E4%B8%B2%E4%B9%98%E6%B3%95/comment-page-1/#comment-122