# 1. Two Sum (Easy)

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

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].
```

## Solutions

``````class Solution {

// Returned values are index of target element, we are required to keep track of the index for each element before
// sorting the nums. Besides, given nums could contain duplicates, so use the Queue to keep the element index.
Map<Integer, Queue<Integer>> map = new HashMap<>();

public int[] twoSum(int[] nums, int target) {
int[] ans = {0, 0};
for (int i = 0; i < nums.length; i++) {
if (!map.containsKey(nums[i])) {
map.put(nums[i], new ArrayDeque<>());
}

}

Arrays.sort(nums);

int i = 0;
int j = nums.length - 1;
while (i < j) {
if (nums[i] + nums[j] < target) {
i++;
}

if (nums[i] + nums[j] > target) {
j--;
}

if (nums[i] + nums[j] == target) {
// Poll it out of the Queue after use just in case the same element appears twice.
ans[0] = map.get(nums[i]).poll();
ans[1] = map.get(nums[j]).poll();

break;
}
}

Arrays.sort(ans);

return ans;
}
}
``````