[Amazon] Big Integer Addition

Given two non-negative integers num1 and num2 represented as string, return the sum of num1 and num2.

 Notice

• The length of both num1 and num2 is < 5100.
• Both num1 and num2 contains only digits 0-9.
• Both num1 and num2 does not contain any leading zero.
• You must not use any built-in BigInteger library or convert the inputs to integer directly

Example

Given num1 = "123", num2 = "45"
return "168"

思路：把长度短的字符串用0补齐位数，从最后一位开始借位相加，反转即可。

分析："123"和"045"

public class Solution {    /**     * @param num1 a non-negative integers     * @param num2 a non-negative integers     * @return return sum of num1 and num2     */    public String addStrings(String num1, String num2) {        StringBuffer sb=new StringBuffer();        int len=Math.max(num1.length(),num2.length());        int carry=0;        for(int i=len-1;i>=0;i--){            if(num1.length()<num2.length()){                for(int j=num1.length();j<num2.length();j++){                    num1="0"+num1;                }            }            if(num1.length()>num2.length()){                for(int j=num2.length();j<num1.length();j++){                    num2="0"+num2;                }            }            char c1=num1.charAt(i);            char c2=num2.charAt(i);            int digit=(carry+(c1-'0')+(c2-'0'))%10;            sb.append(digit);            carry=(carry+(c1-'0')+(c2-'0'))/10;        }                if(carry!=0){            sb.append(carry);        }                return sb.reverse().toString();    }}

第二遍

public class Solution {    /**     * @param num1 a non-negative integers     * @param num2 a non-negative integers     * @return return sum of num1 and num2     */    public String addStrings(String num1, String num2) {        int diff=Math.abs(num1.length()-num2.length());        if(num1.length()>num2.length()){            for(int i=0;i<diff;i++){                num2="0"+num2;            }        }else{            for(int i=0;i<diff;i++){                num1="0"+num1;            }        }                String result="";        int carry=0;        for(int i=num1.length()-1;i>=0;i--){            int sum=carry+(num1.charAt(i)-'0')+(num2.charAt(i)-'0');          //要注意charAt之后要-'0'            int digit=sum%10;            result+=digit;            carry=sum/10;        }                if(carry!=0){            result+=carry;        }        return new StringBuffer(result).reverse().toString();    }}