15. 3Sum

题目描述

Given a binary tree and a sum, determine if the tree has a root-to-leaf path such that adding up all the values along the path equals the given sum.

解题方法

變化馬甲：　two sum == 0 - target

1. 透過Sortu + Two

注意: skip duplicates

    public List<List<Integer>> threeSum(int[] nums){
        List<List<Integer>> lists = new ArrayList<>();
        if(nums == null || nums.length < 3){
            return lists;
        }
        int n = nums.length;
        Arrays.sort(nums);
        for(int i = 0; i < n; i++){
            if(i > 0 && nums[i - 1] == nums[i]){  //skip duplicates //****************************
                continue;
            }

            //two pointer
            int target = nums[i];
            int left = i + 1;
            int right = n - 1;
            while(left < right){
                List<Integer> list = new ArrayList<>();
                if(nums[left] + nums[right] == 0 - target){
                    list.add(nums[left]);
                    list.add(nums[right]);
                    list.add(target);
                    lists.add(list);
                    left++;
                    right--;
                    while(left < right && nums[left] == nums[left - 1]){  //skip duplicates
                        left++;
                    }
                    while(left < right && nums[right] == nums[right + 1]){
                        right--;
                    }
                }else if(nums[left] + nums[right] < 0 - target){
                    left++;
                }else{
                    right--;
                }
            }
        }
        return lists;
    }