LeetCode 67. Add Binary

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Given two binary strings, return their sum (also a binary string).

For example,
a = "11"
b = "1"

Return "100".

一位一位的加就行了，考虑溢出。

public class Solution {    public String addBinary(String a, String b) {        int i = a.length()-1;        int j = b.length()-1;        int flow = 0;        StringBuilder strBuilder = new StringBuilder();        while(i >=0 && j >= 0){            if((a.charAt(i) == '0' || a.charAt(i) == '1')             &&(b.charAt(j) == '0' || b.charAt(j) == '1')){                 int temp = a.charAt(i) - '0' + b.charAt(j) - '0' +flow;                 flow = temp >=2 ? 1 :0;                 temp = temp>=2 ?temp-2 : temp;                 strBuilder.insert(0, temp);             }else{                 return "";             }             i--;             j--;        }        while(i >= 0){             if(a.charAt(i) == '0' || a.charAt(i) == '1'){                 int temp = a.charAt(i) -'0'+flow;                 flow = temp >=2 ? 1 :0;                 temp = temp>=2 ?temp-2 : temp;                  strBuilder.insert(0, temp);              }else{                 return "";             }             i--;        }        while(j >= 0 ){            if(b.charAt(j) == '0' || b.charAt(j) == '1'){                int temp = b.charAt(j) -'0'+flow;                  flow = temp >=2 ? 1 :0;                 temp = temp>=2 ?temp-2 : temp;                  strBuilder.insert(0, temp);             }else{                return "";            }            j--;        }        if(flow == 1) strBuilder.insert(0, "1");        return strBuilder.toString();    }}

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