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].

public class Solution {
    public int[] twoSum(int[] nums, int target) {
        int[] result = new int[2];
        if (nums == null || nums.length == 0) {
            return result;
        }
        HashMap<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            if (map.containsKey(nums[i])) {  // HashMap .containsKey/.containsValue
                result[0] = map.get(nums[i]); // HashMap .get(Object key)
                result[1] = i;
            } else {
                map.put(target - nums[i], i);
            }
        }
        return result;
    }
}
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