Add Binary

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题目

Given two binary strings, return their sum (also a binary string).
For example,
a = "11"
b = "1"
Return "100".
方法

从后往前，每个字符进行判断。
    public String addBinary(String a, String b) {        if (a == null || b == null) {            return null;        }                if (a.equals("")) {            return b;        }        if (b.equals("")) {            return a;        }                int lenA = a.length();        int lenB = b.length();        int i = lenA - 1;        int j = lenB - 1;        boolean flag = false;        StringBuilder builder = new StringBuilder();        while (i >= 0 && j >= 0) {            char curA = a.charAt(i);            char curB = b.charAt(j);            if (flag) {                if (curA == '0' && curB == '0') {                    builder.append(1);                    flag = false;                }                if (curA == '0' && curB == '1') {                    builder.append(0);                    flag = true;                }                if (curA == '1' && curB == '0') {                    builder.append(0);                    flag = true;                }                if (curA == '1' && curB == '1') {                    builder.append(1);                    flag = true;                }            } else {                if (curA == '0' && curB == '0') {                    builder.append(0);                    flag = false;                }                if (curA == '0' && curB == '1') {                    builder.append(1);                    flag = false;                }                if (curA == '1' && curB == '0') {                    builder.append(1);                    flag = false;                }                if (curA == '1' && curB == '1') {                    builder.append(0);                    flag = true;                }                           }            i--;            j--;        }        while (i >= 0) {            char curA  = a.charAt(i);            if (flag) {                if (curA == '0') {                    builder.append(1);                    flag = false;                } else {                    builder.append(0);                    flag = true;                }            } else {                builder.append(curA);            }            i--;        }        while (j >= 0) {            char curB  = b.charAt(j);            if (flag) {                if (curB == '0') {                    builder.append(1);                    flag = false;                } else {                    builder.append(0);                    flag = true;                }            } else {                builder.append(curB);            }            j--;        }        if (flag) {            builder.append(1);        }        return builder.reverse().toString();    }
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