Add Binary - Javacript

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

For example,

a = 

"11"

b = 

"1"

Return 

"100"

.

Tag

Math, String

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/**
* @param {string} a
* @param {string} b
* @return {string}
*/
var addBinary = function(a, b) {
var ret = "";

var len1 = a.length;
var len2 = b.length;

if(len1 === 0 && len2 === 0) {
return ret;
} else if(len1 === 0) {
return b;
} else if(len2 === 0) {
return a;
} else {
if(len1<len2) {
for(var i=0;i<len2-len1;i++) {
a = '0' + a;
}
} else if(len1>len2) {
for(i=0;i<len1-len2;i++) {
b = '0' + b;
}
}

// guarantee that a and b are the same length
var len = a.length;
var carry = 0;

for(i=len-1;i>=0;i--) {
var sum = parseInt(a[i]) + parseInt(b[i]) + carry;
if(sum >= 2) {
carry = 1;
sum = sum % 2;
} else {
carry = 0;
}
ret = sum + ret;
}

if(carry === 1) {
ret = carry + ret;
}

return ret;
}
};