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package practice.map;
import java.util.HashMap;
import java.util.Map;
/**
* Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target.
You may assume that each input would have exactly one solution, and you may not use the same element twice.
You can return the answer in any order.
Example 1:
Input: nums = [2,7,11,15], target = 9
Output: [0,1]
Output: Because nums[0] + nums[1] == 9, we return [0, 1].
Example 2:
Input: nums = [3,2,4], target = 6
Output: [1,2]
Example 3:
Input: nums = [3,3], target = 6
Output: [0,1]
Constraints:
2 <= nums.length <= 103
-109 <= nums[i] <= 109
-109 <= target <= 109
Only one valid answer exists.
来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/two-sum
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。
*/
public class TwoSum {
/**
* 使用HashMap, 一趟遍历, 时间复杂度O(n)，空间复杂度O(n)
* @param nums
* @param target
* @return
*/
public int[] twoSum(int[] nums, int target) {
Map<Integer, Integer> hash = new HashMap<>();
for (int i = 0; i < nums.length; i++) {
int left = target - nums[i];
if (!hash.containsKey(left)) hash.put(nums[i], i);
else return new int[]{i, hash.get(left)};
}
return null;
}
}