15 3Sum

public class Solution {
    public ArrayList<ArrayList<Integer>> threeSum(int[] nums) {
        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
        if (nums == null || nums.length < 3) {
            return res;
        }
        
        for (int i = 0; i < nums.length-2; i++) {
            if (i != 0 && nums[i] == nums[i - 1]) {
                continue;
            }//to skip duplicate numbers [0,0,0,0]
            
            int left = i+1;
            int right = nums.length - 1;
            Arrays.sort(nums);
            while (left < right) {
                int sum = nums[i] + nums[left] + nums[right];
                if (sum == 0) {
                    ArrayList<Integer> arr = new ArrayList<Integer>();
                    arr.add(nums[i]);
                    arr.add(nums[left]);
                    arr.add(nums[right]);
                    res.add(arr);
                    left++;
                    right--;
                    while (left < right && nums[left] == nums[left - 1]) {
                        left++;
                    }//to skip duplicates
                    while (left < right && nums[right] == nums[right + 1]) {
                        right--;
                    }//to skip duplicates
                } else if (sum < 0) {
                    left++;
                } else {
                    right--;
                }
            }
        }
        return res;
    }
}

注意排除重复。