题目：

Given an array of integers, every element appears twice except for one. Find that single one.

代码：

var singleNumber = function(nums) {

var bool = {};

for(var i in nums){

if(!bool[nums[i]]){

bool[nums[i]]=true;

} else {

bool[nums[i]]=false;

}

}

for(var i in nums){

if(bool[nums[i]]){

return nums[i];

}

}

};