leetcode 067 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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class Solution {
public:
    string addBinary(string a, string b) {
        int aLen = a.length(), bLen = b.length(), i=0, index=0;

		if(aLen == 0) return b;
		if(bLen == 0) return a;

		if(aLen < bLen) return addBinary(b, a);

		int cnt=0;
		for(i = bLen-1, index=aLen-1; i >= 0; i--, index--) {
			int temp = a[index]-'0'+b[i]-'0'+cnt;
			cnt = temp/2;
			temp = temp%2;
			a[index] = '0'+temp;
		}

		for(; index >= 0; index--) {
			int temp = a[index]-'0'+cnt;
			cnt = temp/2;
			temp = temp%2;
			a[index] = '0'+temp;
		}

		if(cnt > 0) {
			a = '1'+a;
		}

		return a;
    }
};