leetcode 18. 4Sum

Given an array S of n integers, are there elements a, b, c, and d in S such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.

Note: The solution set must not contain duplicate quadruplets.

For example, given array S = [1, 0, -1, 0, -2, 2], and target = 0.

A solution set is:
[
  [-1,  0, 0, 1],
  [-2, -1, 1, 2],
  [-2,  0, 0, 2]
]

var fourSum = function (nums, target) {
      nums.sort((a, b) => a - b)
      var len = nums.length
      return kSum(nums, len, 0, 4, target)

    };
    function kSum(nums, len, start, k, target) {
      if (k === 2) {
        return twoSum(nums, len, start, target)
      } else {
        var result = []
        for (var i = start; i < len; i++) {
          //不能是第一个
          if (i != start && nums[i] == nums[i - 1]) {
            continue
          }
          var ret = kSum(nums, len, i + 1, k - 1, target - nums[i])
          if (ret.length) {
            ret.forEach(function (el) {
              result.push([nums[i]].concat(el))
            })
          }
        }
        return result
      }
    }
    function twoSum(nums, len, start, target) {
      var left = start;
      var right = len - 1, ret = []
      while (left < right) {
        var sum = nums[left] + nums[right]
        if (sum === target) {
          ret.push([nums[left], nums[right]])
          while (nums[left] == nums[left + 1]) {
            left++
          }
          while (nums[right] == nums[right - 1]) {
            right--
          }
          left++
          right--
        } else if (sum > target) {
          right--
        } else {
          left++
        }
      }
      return ret
    }