LeetCode -- Add Binary

Question:

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

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

Analysis:

思路一：首先将二进制字符串转化成十进制，然后十进制相加，最后再转回二进制。缺点：用int型或long型数据保存变量容易造成溢出，得不到正确结果。

思路二：直接用二进制相加，逢二进一。

Answer：

一：二进制转十进制再转二进制的思路（溢出了/(ㄒoㄒ)/~~）。

public class Solution {
        public String addBinary(String a, String b) {
        if(a.equals("") && b.equals(""))
            return "0";
        if(a == null || a.equals("") || a.equals("0"))
            return b;
        if(b == null || b.equals("") || b.equals("0"))
            return a;
        char[] a1 = a.toCharArray();
        char[] b1 = b.toCharArray();
        long v1 = charToInt(a1);
        long v2 = charToInt(b1);
        long sum = v1 + v2;
        String res = IntToBin(sum);
        return res;
        
    }
    
    private static String IntToBin(long sum) {
        // TODO Auto-generated method stub
        List<Long> l = new ArrayList<Long>();
        while(sum > 0) {
            long t = sum % 2;
            l.add(t);
            sum = sum / 2;
        }
        String res = "";
        for(int i=l.size()-1; i>=0; i--) {
            String t = l.get(i).toString();
            res = res + t;
        }
        
        return res;
    }


    private static int charToInt(char[] a1) {
        // TODO Auto-generated method stub
        int val = 0;
        for(int i=0; i<a1.length; i++) {
            val = val * 2 + (a1[i] - 48);
        }
        return val;
    }
}

二。直接用二进制相加

public class Solution {
    public String addBinary(String a, String b) {
        if(a.equals("") && b.equals(""))
            return "0";
        if(a == null || a.equals("") || a.equals("0"))
            return b;
        if(b == null || b.equals("") || b.equals("0"))
            return a;
        char[] a1 = a.toCharArray();
        char[] b1 = b.toCharArray();
                
        String res = "";
        int i = a1.length - 1;
        int j = b1.length - 1;
        boolean flag = false;
        
        while(i>=0 && j>=0) {
            
            if(a1[i] == '1' && b1[j] == '1' && flag == false) {
                res += '0';
                flag = true;
                i--;
                j--;
                continue;
            }
            if(a1[i] == '1' && b1[j] == '1' && flag == true) {
                res += '1';
                flag = true;
                i--;
                j--;
                continue;
            }
            if(a1[i] == '0' && b1[j] == '1' && flag == true || a1[i] == '1' && b1[j] == '0' && flag == true) {
                res += '0';
                flag = true;
                i--;
                j--;
                continue;
            }
            if(a1[i] == '0' && b1[j] == '1' && flag == false || a1[i] == '1' && b1[j] == '0' && flag == false) {
                res += '1';
                flag = false;
                i--;
                j--;
                continue;
            }    
            if(a1[i] == '0' && b1[j] == '0' && flag == true) {
                res += '1';
                flag = false;
                i--;
                j--;
                continue;
            }
            if(a1[i] == '0' && b1[j] == '0' && flag == false) {
                res += '0';
                flag = false;
                i--;
                j--;
                continue;
            }
            
        }
        
        while(i>=0) {
            if(a1[i] == '0' && flag == true) {
                res += '1';
                flag = false;
            }
            else if(a1[i] == '0' && flag == false) {
                res += '0';
                flag = false;
            }
            else if(a1[i] == '1' && flag == true) {
                res += '0';
                flag = true;
            }
            else if(a1[i] == '1' && flag == false) {
                res += '1';
                flag = false;
            }
            i--;
        }
        
        while(j>=0) {
            if(b1[j] == '0' && flag == true) {
                res += '1';
                flag = false;
            }
            else if(b1[j] == '0' && flag == false) {
                res += '0';
                flag = false;
            }
            else if(b1[j] == '1' && flag == true) {
                res += '0';
                flag = true;
            }
            else if(b1[j] == '1' && flag == false) {
                res += '1';
                flag = false;
            }
            j--;
        }
        
        if(flag == true)
            res += '1';
 
        StringBuffer sb = new StringBuffer(res);
        sb.reverse();
        
        return sb.toString();
    }
}