leetcode刷题笔记十八 四数之和 Scala版本

leetcode刷题笔记十八 四数之和 Scala版本

源地址：18. 四数之和

问题描述：

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.

Note:

The solution set must not contain duplicate quadruplets.

Example:

Given array nums = [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]
]

代码补充：

//import scala.collection.mutable.ListBuffer
object Solution {
    var ansList = List[List[Int]]()
    def fourSum(nums: Array[Int], target: Int): List[List[Int]]= {
    //如果nums的长度小于4个数，返回[]    
    if (nums.length < 4) return List()
    //符合条件的情况下，将ansList重新更新    
    ansList = List[List[Int]]()  
    var sortArr = nums.sorted
    var result =  List[Int]()
    //设置Nlength=4，执行函数
    findNsum(sortArr, target, 4, result)
    return  ansList
  }
  //将4数问题或者说N数之和问题分解为2数之和问题
  //当sortArr.length < Nlength 或者 Nlength < 2的情况时直接返回  
  //当Nlength == 2时，将left设为有序数组的首部，将right设为有序数组的尾部
  //当left < right时，计算sortArr(left)+sort(right)的值与target进行比较，类似与双指针方法
  // 当Nlength > 2时，排除target<Nlength * sortArr(0) 和
  // target > Nlength * sortArr(sortArr.length-1)的情况
  // 针对i分割
    
  def findNsum(sortArr:Array[Int],target:Int,Nlength:Int,result:List[Int]): Unit={  
    if (sortArr.length < Nlength || Nlength < 2) return 
    if (Nlength == 2){
      var left = 0
      var right = sortArr.length - 1
      while (left < right){
        val Tsum = sortArr(left)+sortArr(right)
        Tsum match{
          case sum if (sum < target) => left += 1
          case sum if (sum > target) => right -= 1
          case sum if (sum == target) =>{
          //println ((result++List(sortArr(left),sortArr(right))))
          ansList = ansList :+ result++List(sortArr(left),sortArr(right))
          println(ansList)
          do (left+=1) while(left == 0  || (left < right && sortArr(left) == sortArr(left-1)))
          do (right-=1)while(right>left && sortArr(right) == sortArr(right+1))
          }
        }
      }
    }
    else{
      //println(Nlength)
      for (i <- 0 to sortArr.length-Nlength  if(target >= sortArr(0)*Nlength || target <= sortArr(sortArr.length-1)*4)){
        if(i == 0 || (i > 0 && sortArr(i-1)!=sortArr(i))){
          findNsum(sortArr.slice(i+1,sortArr.length),target-sortArr(i),Nlength-1,result.concat(List(sortArr(i))))
        }
      }
    }
  }
}