Leetcode 15. Sum of three numbers (sort + double pointer)

Wednesday, March 3, 2021, the weather is fine [Do not lament the past, do not waste the present, do not fear the future]

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1. Introduction

Leetcode 15. Sum of Three Numbers

2. Problem solution (sort + double pointer)

The main idea: sorting + double pointer , the difficulty is how to remove duplication .

The algorithm flow is as follows:

class Solution {
    
    
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
    
    
        sort(nums.begin(),nums.end());
        vector<vector<int>> res;
        const int n = nums.size();
        for(int i=0;i<n;++i){
    
    
            // 特判（剪枝）
            if(nums[i]>0) return res;
            // 如果第一个元素重复，提前退出
            if(i>0 && nums[i]==nums[i-1]) continue;

            int beg = i+1, end = n-1;
            while(beg<end){
    
    
                if(nums[beg]+nums[end]==-nums[i]){
    
    
                    // 保存符合条件的三元组
                    res.push_back({
    
    nums[beg],nums[end],nums[i]});

                    // *重点：对第二个和第三个元素进行去重
                    while(beg<end && nums[beg]==nums[beg+1]) ++beg;
                    while(beg<end && nums[end]==nums[end-1]) --end;
                    ++beg;
                    --end;
                }
                else if(nums[beg]+nums[end]>-nums[i])
                    --end;
                else
                    ++beg;
            }
        }
        return res;
    }
};

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references

https://leetcode-cn.com/problems/3sum/solution/pai-xu-shuang-zhi-zhen-zhu-xing-jie-shi-python3-by/