[LeetCode 43] Multiply Strings

题目链接：multiply-strings

import java.util.Arrays;

/**
 * 
		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.
 *
 */

public class MultiplyStrings {

	//解法一
//	311 / 311 test cases passed.
//	Status: Accepted
//	Runtime: 232 ms
//	Submitted: 0 minutes ago

	static String multiply(String num1, String num2) {
        String multi = "";
        int[] nums = new int[num1.length() + num2.length()];
        Arrays.fill(nums, 0);
        for(int i = num1.length() - 1; i >= 0; i --) {
        	for(int j = num2.length() - 1; j >= 0; j --) {
        		nums[i + j + 1] += (num1.charAt(i) - '0') * (num2.charAt(j) - '0');
        	}
        }
		for (int i = nums.length - 1; i >= 1; i--) {
			nums[i - 1] += nums[i] / 10;
			nums[i] %= 10;
		}
    	int begin = 0;
    	while(begin < nums.length - 1 && nums[begin] == 0) begin ++;
		for (int i = begin; i <= nums.length - 1; i ++) {
				multi += nums[i];
		}
    	return multi;
    }

	
	
	//解法二
//	311 / 311 test cases passed.
//	Status: Accepted
//	Runtime: 394 ms
//	Submitted: 0 minutes ago
	//代码略显臃肿，有待优化
	static String multiply1(String num1, String num2) {
        String multi = "";
        String temp = num2;
        for(int i = num1.length() - 1; i >= 0; i --) {
        	int n = num1.charAt(i) - '0';
        	multi = add(multi, multiply(temp, n));
        	temp += "0";
        }
    	int begin = 0;
    	while(begin < multi.length() - 1 && multi.charAt(begin) == '0') begin ++;
    	return multi.substring(begin);
    }
    
    static String multiply(String num1, int num2) {
    	String multi = "";
    	if(num2 == 0) return "0";
        int carray = 0;
        for(int i = num1.length() - 1; i >= 0; i --) {
        	int n = (num1.charAt(i) - '0') * num2 + carray;
        	multi = n % 10 + multi;
        	carray = n / 10;
        }
    	if(carray != 0) {
    		multi = carray % 10 + multi;
    	}
    	return multi;
    }
    static String add(String a, String b) {
    	String sum = "";
        int carray = 0;
        if(a.length() > b.length()) {
        	String temp = a;
        	a = b;
        	b = temp;
        }
        //使a和b的长度相等
        int gap = b.length() - a.length();
        while((gap--) != 0) a = "0" + a;
        
    	for (int i = b.length() - 1; i >= 0; i--) {
			int n = a.charAt(i) - '0' + b.charAt(i) - '0' + carray;
			sum = (n % 10) + sum;
			carray = n / 10;
		}
    	if(carray != 0) sum = carray % 10 + sum;
    	return sum;
    }
	public static void main(String[] args) {
		System.out.println(multiply("1000000", "99999"));

		System.out.println(multiply("1000000000000000000000000000000000000000000", "1000000000000000000000000000000000000000000"));
		
	}

}

[LeetCode 43] Multiply Strings

原文：http://blog.csdn.net/ever223/article/details/44587173