public List<List<Integer>> threeSum(int[] nums) {//1.先排序后好用三指针Arrays.sort(nums);int k = 0;//2.特判：第一个数>0，则无；if (nums[k] > 0) {//注意不能直接返回nullreturn new ArrayList<List<Integer>>();}List<List<Integer>> lists = new ArrayList<>();//3.当k在倒数第二个位置时，则可以直接returnwhile (k <= nums.length - 3) {int i=k+1;int j = nums.length-1;//while循环注意越界while (k > 0&&k<nums.length&& nums[k] == nums[k - 1]) {k++;}//注意加上下面合适的if判断，不然【0,0,0,0】出问题if(k>=nums.length-2){return lists;}while (i < j) {//此处不应该为if,且应该加个i<j，不然【0,0,0】出问题while(i<j&&(nums[k] + nums[i] + nums[j]) == 0) {lists.add(new ArrayList<Integer>(Arrays.asList(nums[i],nums[j],nums[k])));//nums[++i] == nums[i]相当于nums[i+1] == nums[i+1]；i++；没赋值，必然的while (i<j&&nums[i] == nums[++i]) ;while (j>i&&nums[j] == nums[--j]) ;}//(下面两个while需要判断i,j关系)while (j>i&&(nums[k] + nums[i] + nums[j]) < 0) i++;while (j>i&&(nums[k] + nums[i] + nums[j]) > 0) j--;}k++;}return lists;
class Solution {public List<List<Integer>> threeSum(int[] nums) {Arrays.sort(nums);List<List<Integer>> res = new ArrayList<>();for(int k = 0; k < nums.length - 2; k++){if(nums[k] > 0) break;if(k > 0 && nums[k] == nums[k - 1]) continue;int i = k + 1, j = nums.length - 1;while(i < j){int sum = nums[k] + nums[i] + nums[j];if(sum < 0){while(i < j && nums[i] == nums[++i]);} else if (sum > 0) {while(i < j && nums[j] == nums[--j]);} else {res.add(new ArrayList<Integer>(Arrays.asList(nums[k], nums[i], nums[j])));while(i < j && nums[i] == nums[++i]);while(i < j && nums[j] == nums[--j]);}}}return res;}
}