LeetCode 67 : Add Binary (Java)

解题思路：从末尾依次对应相加来求，转成int中的相加会使代码比较简洁，因为可以用/和%来分别求出进位和当前往结果中添加的位。然后，把较长的字符串剩下的部分和进位相加放入结果。最后，还要判断进位此时是否为1，如果为1，则需要再往结果中添加1，否则返回结果。

public class Solution {
public String addBinary(String a, String b) {
if(a.length() < b.length()){
String tmp = a;
a = b;
b = tmp;
}

int pa = a.length()-1;
int pb = b.length()-1;
int carries = 0;
String rst = "";

while(pb >= 0){
int sum = (int)(a.charAt(pa) - '0') + (int)(b.charAt(pb) - '0') + carries;
rst = String.valueOf(sum % 2) + rst;
carries = sum / 2;
pa --;
pb --;
}

while(pa >= 0){
int sum = (int)(a.charAt(pa) - '0') + carries;
rst = String.valueOf(sum % 2) + rst;
carries = sum / 2;
pa --;
}

if (carries == 1)
rst = "1" + rst;
return rst;
}
}