(Java)LeetCode-43. Multiply Strings

最新推荐文章于 2019-07-07 13:33:21 发布

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最新推荐文章于 2019-07-07 13:33:21 发布

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分类专栏： JAVA LeetCode 文章标签： string Math

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本文介绍了一种不使用BigInteger等内置库实现两个大数字符串相乘的方法。通过模拟手写计算过程，避免了直接转换为整数进行计算，适用于任意大小的非负整数相乘。

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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.
Converting the input string to integer is NOT allowed.

You should NOT use internal library such as BigInteger.

这道题蛮简单的，算两个大数的乘法，模拟手算时的竖式计算即可，需要做MN次乘法和若干次加法，需要耐心~代码如下：

public class Solution {
    
	
	public String multiply(String num1, String num2) {
		int len1 = num1.length();
		int len2 = num2.length();
		if(num1.equals("0")|| num2.equals("0"))
		    return "0";
		int len = len1 + len2-1;
		int carry = 0;
		int temp = 0, templeft = 0, tempright = 0;
		int sum = 0;
		Stack<Integer> st = new Stack<Integer>();
		for(int i = 1; i <= len; i++){
			for(int j = 1; j <= i; j++){
				if( i+1-j > len2 || j > len1){
					continue;
				}
				templeft = Integer.parseInt(num1.substring(len1-j, len1-j+1)); 
				tempright = Integer.parseInt(num2.substring(len2-1-i+j, len2-i+j));
				temp = templeft * tempright;
				sum = temp + sum;
			}
			sum +=carry;
			st.push(sum % 10);
			carry = sum/10;
			sum = 0;
		}
		while(carry != 0){
			st.push(carry % 10);
			carry /= 10;
		}
		StringBuilder sb = new StringBuilder();
		while(!st.isEmpty()){
			sb.append(st.pop());
		}
		return sb.toString();
    } 

}