模板化了,要计算 k-sum 复杂度为O(N^(k-1))
package Level3; import java.util.ArrayList; import java.util.Arrays; import java.util.HashSet; import java.util.Set; /** * 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: Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a ≤ b ≤ c ≤ d) 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) */ public class S18 { public static void main(String[] args) { int[] num = {1, 0, -1, 0, -2, 2}; System.out.println(fourSum(num, 0)); } // O(n^3) public static ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) { Set<ArrayList<Integer>> ret = new HashSet<ArrayList<Integer>>(); Arrays.sort(num); for (int i = 0; i < num.length; i++) { for (int j = i+1; j < num.length; j++) { int m = j+1, n = num.length-1; while(m < n){ if(i!=j && j!=m && m!=n){ if(num[m] + num[n] == target-num[i]-num[j]){ ArrayList<Integer> list = new ArrayList<Integer>(); list.addAll(Arrays.asList(num[i], num[j], num[m], num[n])); ret.add(list); m++; n--; }else if(num[m] + num[n] < target-num[i]-num[j]){ m++; }else{ n--; } } } } } return new ArrayList<ArrayList<Integer>>(ret); } // O(n^3 * log(n)) 超时! public static ArrayList<ArrayList<Integer>> fourSum2(int[] num, int target) { Set<ArrayList<Integer>> ret = new HashSet<ArrayList<Integer>>(); Arrays.sort(num); for (int i = 0; i < num.length; i++) { for (int j = num.length-1; j > i+1; j--) { for (int k = i+1; k < j; k++) { if(i!=j && j!=k){ int remain = target - (num[i] + num[j] + num[k]); int index = Arrays.binarySearch(num, k + 1, j, remain); if (index >= 0) { ArrayList<Integer> list = new ArrayList<Integer>(); list.add(num[i]); list.add(num[k]); list.add(num[index]); list.add(num[j]); ret.add(list); } } } } } return new ArrayList<ArrayList<Integer>>(ret); } }
public class Solution { public List<List<Integer>> fourSum(int[] num, int target) { List<List<Integer>> ret = new ArrayList<List<Integer>>(); int len = num.length; Arrays.sort(num); for(int i=0; i<len; i++) { if(i >= 1 && i < len && num[i] == num[i-1]) { continue; } for(int j=i+1; j<len; j++) { if(j >= i+2 && j < len && num[j] == num[j-1]) { continue; } int left = j+1, right = len-1; while(left < right) { int sum = num[i] + num[j] + num[left] + num[right]; if(sum == target) { List<Integer> al = new ArrayList<Integer>(); al.add(num[i]); al.add(num[j]); al.add(num[left]); al.add(num[right]); ret.add(al); do { left++; } while(left < right && num[left] == num[left-1]); do { right--; } while(left < right && num[right] == num[right+1]); } else if(sum < target) { left++; } else { right--; } } } } return ret; } }