Leetcode-15 3Sum

三数之和

最新推荐文章于 2022-02-10 12:14:07 发布

原创最新推荐文章于 2022-02-10 12:14:07 发布 · 157 阅读

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本文介绍了一种解决三数之和问题的有效算法。该算法首先对输入数组进行排序，然后通过固定一个元素并使用双指针技术来寻找剩余两个元素，确保它们的和等于零。此方法避免了重复解，并通过合理的剪枝策略提高了搜索效率。

Given an array nums of n integers, are there elements a, b, c in numssuch that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

Note:

The solution set must not contain duplicate triplets.

Example:

Given array nums = [-1, 0, 1, 2, -1, -4],

A solution set is:

[

[-1, 0, 1],

[-1, -1, 2]

]

方法一、先排序，再fix一个number。注意各种剪枝！

import static java.util.Arrays.asList;
class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        List<List<Integer>> result_list = new ArrayList<>();
        Qsort(nums, 0, nums.length-1);
        for (int i=0; i<nums.length; i++){
            if (nums[i]>0){
                break;
            }
            if (i>0 && nums[i]==nums[i-1]){
                continue;
            }
            int target = 0-nums[i];
            int start = i+1;
            int end = nums.length-1;
            while(start < end){
                if(nums[start] + nums[end] == target){
                    result_list.add(asList(nums[i],nums[start],nums[end]));
                    while(start<end && nums[start]==nums[start+1]){
                        start++;
                    }
                    while(start<end && nums[end] == nums[end-1]){
                        end--;
                    }
                    start++;
                    end--;
                }
                else if(nums[start]+nums[end]<target){
                    start++;
                }
                else{
                    end--;
                }
                
                
            }
            
        }
        return result_list;
    }
    public static void Qsort(int[] nums, int first, int last){
        if(first>=last){
            return;
        }
        int first_index=first;
        int last_index=last;
        int key = nums[first];
        while(first_index<last_index){
            while(first_index<last_index && nums[last_index]>=key){
                last_index--;
            }
            nums[first_index] = nums[last_index];
            
            while(first_index<last_index && nums[first_index]<=key){
                first_index++;
            }
            nums[last_index] = nums[first_index];
        }
        nums[first_index] = key;
        
        Qsort(nums, first, first_index);
        Qsort(nums, first_index+1, last);
    }
}