leecode algo1: Two Sum (Java)

leecode algo1: Two Sum

題目：

Given an array of integers, find two numbers such that they add up to a specific target number.

The function twoSum should return indices of the two numbers such that they add up to the target, where index1 must be less than index2. Please note that your returned answers (both index1 and index2) are not zero-based.

You may assume that each input would have exactly one solution.

Input: numbers={2, 7, 11, 15}, target=9
Output: index1=1, index2=2

具體實現如下（leetcode 提交通過， Run Time：7ms）：

package algo1;

import java.util.HashMap;

public class Solution {

	public static void main(String[] args) {
		Solution s = new Solution();
		int[] nums = {2, 1, 9, 4, 4, 56, 90, 3};
		int[] res = s.twoSum(nums, 8);
		System.out.println(res[0] + "\t" + res[1]);
	}

	public int[] twoSum(int[] nums, int target) {
		HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
		for(int i = nums.length - 1; i > 0; i--) {
			if(target == nums[i] * 2) {
				if(map.containsKey(nums[i])) {
					int[] res = new int[2];
					res[0] = i + 1;
					res[1] = map.get(nums[i]) + 1;
					return res;
				} else {
					map.put(nums[i], i);
				}
			} else {
				map.put(nums[i], i);
			}
		}
		for(int i = 0; i < nums.length; i++) {
			if(map.containsKey(target - nums[i])) {
				int[] res = new int[2];
				res[0] = i + 1;
				res[1] = map.get(target - nums[i]) + 1;
				return res;
			}
		}
		return null;
	}
}