【leetcode刷题笔记】Add Binary

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

For example,
a = "11"
b = "1"
Return "100".

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题解：简单的二进制加法模拟。a，b的最后以为对齐开始进行加法，用carries保存进位，如果加完后最高位还有进位，那么要在结果的最前面加一个1。

代码如下：

public class Solution {
    public String addBinary(String a, String b) {
        if(a == null || a.length() == 0)
            return b;
        if(b == null || b.length() == 0)
            return a;
        
        String s = new String();
        int carries = 0;
        int a_kepeler = a.length()-1;
        int b_kepeler = b.length()-1;
        
        while(a_kepeler >= 0 && b_kepeler >= 0){
            int sum = carries + a.charAt(a_kepeler) - '0' + b.charAt(b_kepeler) - '0';
            carries = sum / 2;
            sum = sum % 2;
            s = String.valueOf(sum) + s;
            a_kepeler--;
            b_kepeler--;
        }
        
        while(a_kepeler >= 0){
            int sum = carries + a.charAt(a_kepeler) - '0';
            carries = sum / 2;
            sum = sum % 2;
            s = String.valueOf(sum) + s;
            a_kepeler--;
        }
        
        while(b_kepeler >= 0){
            int sum = carries + b.charAt(b_kepeler) - '0';
            carries = sum / 2;
            sum = sum % 2;
            s = String.valueOf(sum) + s;
            b_kepeler--;
        }
        
        if(carries > 0)
            s = "1" + s;
        
        return s;
    }
}

上述代码还可以优化，就是如果a的长度小于b，就把a,b交换，使得a总是较长的那个，那么就可以省略第30~36行的while循环了。