Leetcode||15.3Sum

15. 3Sum

• Total Accepted: 158805
• Total Submissions: 777436
• Difficulty: Medium
• Contributors: Admin

Given an array S of n integers, are there elements a,b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

Note: The solution set must not contain duplicate triplets.

For example, given array S = [-1, 0, 1, 2, -1, -4],A solution set is:[  [-1, 0, 1],  [-1, -1, 2]]

这题是看着网上的解法，结果还是暴力枚举，剪枝不太会，有解法

代码如下：

暴力枚举

public class Solution {    public List<List<Integer>> threeSum(int[] nums) {        List<List<Integer>> rst = new ArrayList<List<Integer>>();Arrays.sort(nums);for(int i = 0; i < nums.length-2; i++){              if (nums[i] > 0) {                  return rst;              }              if(i>=1 && nums[i] == nums[i-1]){                  continue;              }              for(int j = i+1; j < nums.length-1; j++){                  if(j>= i+2 && nums[j] == nums[j-1]){                      continue;                  }                  for(int k = j+1 ; k < nums.length; k++){                      if(k>=j+2 && nums[k] == nums[k-1]){                          continue;                      }                      if(nums[i]+nums[j]+nums[k]<0){                          continue;                      }                      else if(nums[i]+nums[j]+nums[k] == 0){                          List<Integer> temp = Arrays.asList(nums[i],nums[j],nums[k]);                          rst.add(temp);                      }else break;                  }              }          }          return rst;    }}

剪枝法

public class Solution {    public List<List<Integer>> threeSum(int[] nums) {        List<List<Integer>> result = new ArrayList<>();      int length = nums.length;      if (length < 3) {          return result;      }      Arrays.sort(nums);      for (int i = 0; i < length - 2; i++) {          if (nums[i] > 0) {              return result;          }          if (i == 0 || nums[i] != nums[i - 1]) {              int target = -nums[i];              int low = i + 1, high = length - 1;              while (low < high) {                  if (nums[high] < 0 || nums[low] > target) {                      break;                  }                  if (nums[low] + nums[high] == target) {                      result.add(Arrays.asList(nums[i], nums[low], nums[high]));                      while(++low<high && nums[low] == nums[low-1]);                      while(low<--high && nums[high] == nums[high+1]);                  } else if (nums[low] + nums[high] < target) {                      while(++low<high && nums[low] == nums[low-1]);                  } else {                      while(low<--high && nums[high] == nums[high+1]);                  }              }          }      }      return result;     }}