15. 3Sum

public class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        List<List<Integer>> results = new ArrayList<>();
        if( nums == null || nums.length <= 2){
            return results;
        }
        Arrays.sort(nums);
        for(int i = 0; i < nums.length - 2; i++){
            if(i!= 0 && nums[i] == nums[i - 1]){
                continue;
            }
            int left = i+1;
            int right = nums.length - 1;
            while(left < right){
                if(nums[i] + nums[left] + nums[right] > 0){
                    right --;
                    
                    }else if(nums[i] + nums[left] + nums[right] == 0){
                    ArrayList <Integer> list = new ArrayList<>();
                    list.add(nums[i]);
                    list.add(nums[left]);
                    list.add(nums[right]);
                    results.add(list);
                    left ++;
                    right --;
                    while(left < right && nums[left] == nums[left - 1]){// 关键点！！！
                        left ++;
                    }
                    while(right > left  && nums[right] == nums[right + 1]){
                        right --;
                    }
                }else{
                    left ++;
                   
                }
            }
        }
        return results;
    }
}