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import java.util.Arrays;

import java.util.HashMap;

import java.util.Map;

/**

* LC#1：Two Sum 两数之和

* https://leetcode-cn.com/problems/two-sum/

* 思路1：2次哈希求解：将数组放入散列表，遍历散列表，通过target-key得出匹配数组，并且判断非自身元素，返回数组，时间复杂度 O（n）

* 思路2[runtime beats（最优解）]：1次哈希求解：在遍历 num 数组，如果哈希表不包含 target - num[i] ，则将 num[i], i 放入哈希表，如果包含则直接返回 [hash[num[i], i]] 下标

*/

public class S1 {

public static int[] twoSum(int[] nums, int target) {

Map<Integer, Integer> hashMap = new HashMap<>();

for (int i = 0; i < nums.length; i++) {

// 如果包含目标值

if(hashMap.containsKey(target - nums[i])) {

return new int[]{hashMap.get(target - nums[i]), i};

}

hashMap.put(nums[i], i);

}

return null;

}

public static void main(String[] args) {

int[] nums = {2, 7, 11, 15};

int target = 9;

int[] result = S1.twoSum(nums, target);

System.out.println(Arrays.toString(result));

}

}