Algorithms—43.Multiply Strings

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思路：按照乘法的定义，逐位相乘。

public class Solution {    public String multiply(String num1, String num2) {        if (num1.equals("0")||num2.equals("0")) {return "0";}    if (num1.length()<num2.length()) {return multiply(num2,num1);}    int l1=num1.length();        int[] n1=new int[l1];        for (int i = 0; i <l1; i++) {n1[l1-i-1]=Integer.parseInt(num1.substring(i, i+1));}        int l2=num2.length();        int[] n2=new int[l1];        for (int i = 0; i <l2; i++) {n2[l2-i-1]=Integer.parseInt(num2.substring(i, i+1));}        int[] a=new int[l1+l2-1];    int sum=0;    int carry=0;for (int i = 0; i < a.length; i++) {// 每一位乘级相加的结果for (int j = Math.max(0,i-l2+1); j <= Math.min(i,l1-1); j++) {sum += n1[j] * n2[i-j];}// 加进位sum += carry;carry = sum / 10;sum%=10;a[i]=sum;sum = 0;}StringBuffer sb=new StringBuffer();if (carry!=0) {sb.append(String.valueOf(carry));}    for (int i = a.length-1; i >=0 ; i--) {sb.append(String.valueOf(a[i]));}    return sb.toString();    }}

耗时：368ms，中下游。

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