1.two-sum.java

/*
 * [1] Two Sum
 *
 * https://leetcode.com/problems/two-sum
 *
 * algorithms
 * Easy (36.39%)
 * Total Accepted:    723.8K
 * Total Submissions: 2M
 * Testcase Example:  '[3,2,4]\n6'
 *
 * Given an array of integers, return indices of the two numbers such that they
 * add up to a specific target.
 * 
 * You may assume that each input would have exactly one solution, and you may
 * not use the same element twice.
 * 
 * 
 * Example:
 * 
 * Given nums = [2, 7, 11, 15], target = 9,
 * 
 * Because nums[0] + nums[1] = 2 + 7 = 9,
 * return [0, 1].
 * 
 * 
 */
class Solution {
    public int[] twoSum(int[] nums, int target) {
	if (nums.length <= 1) {
		return new int[]{};
	}
	
	Map<Integer, Integer> indexMap = new HashMap<>();
	for (int i = 0; i < nums.length; i++) {
		int complement = target - nums[i];
		if (indexMap.containsKey(complement)) {
			return new int[]{indexMap.get(complement), i};
		}
		indexMap.put(nums[i], i);
	}
	return new int[]{};
    }
}