15. 3Sum

给出数组nums，判断 nums是否存在不相等的三个元素 a, b, c，使得其结果为0，返回结果数组。

• Runtime: 136 ms, faster than 97.33%
• Memory Usage: 48.7 MB, less than 87.40%

/**
 * @param {number[]} nums
 * @return {number[][]}
 */
var threeSum = function(nums) {
    var n = nums.length
    var res = []
    if (n < 3) return res
    nums.sort((a, b) => {
        return a - b
    })
    for (var i = 0; i < n - 2; i++) {
        if(i > nums[0] && nums[i] === nums[i - 1] ) continue
        var j = i + 1
        var k = n - 1
        while (j < k) {
            if (nums[i] + nums[j] + nums[k] < 0){
                j++
                while(nums[j] === nums[j - 1] && j < k) {
                    j++
                }
            } else if(nums[i] + nums[j] + nums[k] > 0){
                k --
                while(nums[k] === nums[k + 1] && j < k) {
                    k--
                }
            } else{
                res.push([nums[i], nums[j], nums[k]])
                j++
                k--
                while(nums[j] === nums[j - 1]) {
                    j++
                }
            }
        } 
    }
    return res
};