1.题目 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] ]
2.解法
public class Solution { public List<List<Integer>> fourSum(int[] nums, int target) { List arr = new ArrayList(); int size = nums.length; if(size < 4){ return arr; } Arrays.sort(nums); for(int i = 0; i < size - 3; i++){ // pruning if(i > 0 && nums[i] == nums[i-1]) continue; if(nums[i] + nums[i+1] + nums[i+2] + nums[i+3] > target) break; if(nums[i] + nums[size - 1] + nums[size - 2] + nums[size- 3] < target) continue; for(int j = i + 1; j < size - 2; j++){ if(j > i + 1 && nums[j] == nums[j - 1]) continue; if(nums[i] + nums[j] + nums[j+1] + nums[j+2] > target) break; if(nums[i] + nums[j+1] + nums[size -1] + nums[size- 2] < target) continue; int left = j + 1, right = size - 1; while(left < right){ int sum = nums[i] + nums[j] + nums[left] + nums[right]; if(sum > target){ --right; }else if(sum < target){ ++left; }else{ arr.add(new int[]{nums[i], nums[j], nums[left], nums[right]}); do{ ++left; }while(nums[left] == nums[left - 1] && left < right); do{ --right; }while(nums[right] == nums[right + 1] && left < right); } } } } return arr; } }