题目

给定两个二进制字符串，返回他们的和（用二进制表示）。
输入为非空字符串且只包含数字 1 和 0。

示例

示例 1:

输入: a = “11”, b = “1”
输出: “100”

示例 2:

输入: a = “1010”, b = “1011”
输出: “10101”

思路

    这题其实从后往前遍历就行，只是需要在短的数字串前面补0，否则就会报错，注意进制的问题就行：

public String addBinary(String a, String b) {
        StringBuilder newA=new StringBuilder(a);
        StringBuilder newB=new StringBuilder(b);
        StringBuilder temp=new StringBuilder();
        int length=newA.length() > newB.length() ? newA.length() - newB.length():newB.length()-newA.length();
        if (newA.length() > newB.length()){
            while (length-- > 0){
                newB.insert(0,"0");
            }
        }else {
            while (length-- > 0){
                newA.insert(0,"0");
            }
        }//        System.out.println(newA+" "+newB);
        length=newA.length();
        int flag=0;
        for (int i=length-1; i >= 0;i--){
            if(newA.charAt(i) == '0' && newB.charAt(i) == '1' || newA.charAt(i) == '1' && newB.charAt(i) == '0'){
                if (flag == 1){
                    temp.insert(0,"0");
                    flag=1;
                }else {
                    temp.insert(0,"1");
                    flag=0;
                }
            }else if(newA.charAt(i) == '0' && newB.charAt(i) == '0'){
                if (flag == 1){
                    temp.insert(0,"1");
                    flag=0;
                }else {
                    temp.insert(0,"0");
                    flag=0;
                }
            }else {
                if (flag == 1){
                    temp.insert(0,"1");
                    flag=1;
                }else {
                    temp.insert(0,"0");
                    flag=1;
                }
            }
        }
        if (flag == 1){
            temp.insert(0,"1");
        }
        return temp.toString();
    }