(java)Leetcode-15. 3Sum

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:

• Elements in a triplet (a,b,c) must be in non-descending order. (ie, a ≤ b ≤ c)
• 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)

这道题我是想暴力算法的，就是弄三个循环，注意去除重即可，代码如下：

public List<List<Integer>> threeSum(int[] nums) {        Arrays.sort(nums);           List<List<Integer>> result = new ArrayList<List<Integer>>();    for(int i = 0; i < nums.length-2; i++){    if (nums[i] > 0) {                return result;            }    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]);    result.add(temp);    }else break;    }    }    }    return result;          }        public static void main(String[] args){    Solution sol = new Solution();    int[] aa = {-1,0,1,2,-1,-4};    List<List<Integer>> result = sol.threeSum(aa);    for(List<Integer> list : result){    for(int in : list){    System.out.print(in+" ");    }    System.out.println();    }    }

不过其实有很多情况可以排除不去考虑，从而节省很多时间。比如下面这个就很快了，思路很巧妙。

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;    }