package LeetCode_18
/**
* 18. 4Sum
* https://leetcode.com/problems/4sum/
* Given an array nums of n integers and an integer target,
* are there elements a, b, c, and d in nums such that a + b + c + d = target?
* Find all unique quadruplets in the array which gives the sum of target.
Notice that the solution set must not contain duplicate quadruplets.
Example 1:
Input: nums = [1,0,-1,0,-2,2], target = 0
Output: [[-2,-1,1,2],[-2,0,0,2],[-1,0,0,1]]
Example 2:
Input: nums = [], target = 0
Output: []
Constraints:
1. 0 <= nums.length <= 200
2. -109 <= nums[i] <= 109
3. -109 <= target <= 109
* */
class Solution {
/*
* solution: base on 3Sum, Time:O(n^3), Space:O(1)
* */
fun fourSum(nums: IntArray, target: Int): List<List<Int>> {
val result = ArrayList<ArrayList<Int>>()
if (nums.size < 4) {
return result
}
nums.sort()
val n = nums.size
for (i in 0 until n - 3) {
//avoid duplicate
if (i > 0 && nums[i] == nums[i - 1]) {
continue
}
for (j in i + 1 until n - 2) {
//avoid duplicate
if (j > i + 1 && nums[j] == nums[j - 1]) {
continue
}
var left = j + 1
var right = n - 1
while (left < right) {
val sum = nums[i] + nums[j] + nums[left] + nums[right]
if (sum == target) {
//find out one result
result.add(arrayListOf(nums[i], nums[j], nums[left], nums[right]))
//avoid duplicate
while (left < right && nums[left] == nums[left + 1]) {
left++
}
while (left < right && nums[right] == nums[right - 1]) {
right--
}
left++
right--
} else if (sum < target) {
//if sum less than target, we need more large number, so left++, because array was sorted
left++
} else {
right--
}
}
}
}
return result
}
}
3Sum
