[leetcode]Add Binary

Add Binary

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

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

算法思路：

模拟二进制加法，跟十进制木有区别，将a，b转置(不转置的话，倒着遍历也是同样道理)，这样就可以从低位到高位逐级递加了

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;
        // assume that b.length is longer
        if(b.length() < a.length()) return addBinary(b, a);
        StringBuilder br = new StringBuilder(b).reverse();
        StringBuilder ar = new StringBuilder(a).reverse();
        int leap = 0;
        for(int i = 0; i < br.length(); i++){
               int aNum = (i < ar.length())? (ar.charAt(i) == '0' ? 0 : 1) : 0;
            int bNum = br.charAt(i) == '0' ? 0 : 1;
            if(aNum + bNum + leap == 1){
                br.setCharAt(i, '1');
                leap = 0;
            }else if(aNum + bNum + leap == 2){
                br.setCharAt(i, '0');
                leap = 1;
            }else if(aNum + bNum + leap == 3){
                br.setCharAt(i, '1');
                leap = 1;
            }
        }
        if(leap == 1) br.append('1');
        return br.reverse().toString();
    }
}