数组 - 只出现一次的数字

/**
 * @param {number[]} nums
 * @return {number}
 */
var singleNumber = function(nums) {
    var cache = {};
    var len = nums.length;
    
    for (var i = 0 ; i < len ; i++) {
        cache[nums[i]] = cache[nums[i]] ? (cache[nums[i]] + 1) : 1;
    }
    
    for (var k in cache) {
        if (cache[k] === 1) {
            return k;
        }
    }
    
    return 0;
};

/**
 * @param {number[]} nums
 * @return {number}
 */
var singleNumber = function (nums) {
    var res;
    nums.forEach(function (v) {
        res = res ^ v;
    });
    return res;
};