LeetCode 算法 每日一题 1.两数之和

1.两数之和

题目描述

给定一个整数数组 nums 和一个整数目标值 target，请你在该数组中找出 和为目标值 的那 两个 整数，并返回它们的数组下标。
你可以假设每种输入只会对应一个答案。但是，数组中同一个元素在答案里不能重复出现。
你可以按任意顺序返回答案。
示例1

输入：nums = [2,7,11,15], target = 9
输出：[0,1]
解释：因为 nums[0] + nums[1] == 9 ，返回 [0, 1] 。

示例2

输入：nums = [3,2,4], target = 6
输出：[1,2]

示例3

输入：nums = [3,3], target = 6
输出：[0,1]

提示

2 <= nums.length <= 103
-109 <= nums[i] <= 109
-109 <= target <= 109
只会存在一个有效答案

使用暴力枚举方法提出所有的数字对，尝试暴力解决方案并优化，但是速度过慢。
搜索(x,y)与搜索target - x 的速度差别
在不使用数组情况下，也可以使用图（散列图、哈希）来解决问题

代码演示
C++11 暴力枚举

#include <bits/stdc++.h>
using namespace std;

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        int n = nums.size();
        int i, j;
        for (i = 0; i < n; ++i) {
            for (j = i + 1; j < n; ++j) {
                //target-num[i] == num[j]
                if (nums[i] + nums[j] == target) {
                    return {i,j};
                }
            }
        }
        return {};
    }
};

int main()
{
    int array1[4] = {2,7,11,15};
    int array2[3] = {3,2,4};
    int array3[2] = {3,3};

    int target1 = 9;
    int target2 = 6;
    int target3 = 6;

    Solution s;
    vector<int> nums1(array1,array1 + 4);
    vector<int> result = s.twoSum(nums1,target1);
    cout << "[" << *result.begin() << "," << *(result.begin()+1) << "]" << endl;
    vector<int> nums2(array2,array2 + 3);
    result = s.twoSum(nums2,target2);
    cout << "[" << *result.begin() << "," << *(result.begin()+1) << "]" << endl;
    vector<int> nums3(array3,array3 + 2);
    result = s.twoSum(nums3,target3);
    cout << "[" << *result.begin() << "," << *(result.begin()+1) << "]" << endl;

    return 0;
}

代码演示
Java 哈希

import java.util.HashMap;

public class OneQuestionPerDay1 {
    public static void main(String[] args) {
       
        int num01[] = {2,7,11,15};int target01 = 9;

        int num02[] = {3,2,4};int target02 = 6;

        int num03[] = {3,3};int target03 = 6;

        int num04[] = {1,3,5,7};int target04 = 9;

        Solution s = new Solution();
        int result[] = new int[2];

        result = s.twoSum(num01, target01);
        System.out.println("[" + result[0]+ "," + result[1] + "]");

        result = s.twoSum(num02, target02);
        System.out.println("[" + result[0]+ "," + result[1] + "]");

        result = s.twoSum(num03, target03);
        System.out.println("[" + result[0]+ "," + result[1] + "]");

        System.out.println(s.twoSum(num04,target04).length);

    }
}

class Solution{
    public int[] twoSum(int[] nums,int target) {
        HashMap<Integer,Integer> map = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            if(map.containsKey(target - nums[i]))
                return new int[] {map.get(target - nums[i]),i};
            map.put(nums[i],i);
        }
        return new int[0];
    }
}

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