• Time:O(n)
• Space:O(n)

## C++

``````class Solution {
public:
vector<int> twoSum(vector<int>& nums, int target) {
unordered_map<int, int> numToIndex;

for (int i = 0; i < nums.size(); ++i) {
if (numToIndex.count(target - nums[i]))
return {numToIndex[target - nums[i]], i};
numToIndex[nums[i]] = i;
}

throw;
}
};
``````

## JAVA

``````class Solution {
public int[] twoSum(int[] nums, int target) {
Map<Integer, Integer> numToIndex = new HashMap<>();

for (int i = 0; i < nums.length; ++i) {
if (numToIndex.containsKey(target - nums[i]))
return new int[] {numToIndex.get(target - nums[i]), i};
numToIndex.put(nums[i], i);
}

throw new IllegalArgumentException();
}
}
```

Solution-2
```
class Solution {
public int[] twoSum(int[] nums, int target) {
HashMap<Integer, Integer> map = new HashMap<>();
int i=0;
for(; !map.containsKey(nums[i]); i++) {
map.put(target - nums[i], i);
}
return new int[]{map.get(nums[i]), i};
}
}
``````

## Python

``````class Solution:
def twoSum(self, nums: List[int], target: int) -> List[int]:
numToIndex = {}

for i, num in enumerate(nums):
if target - num in numToIndex:
return numToIndex[target - num], i
numToIndex[num] = i
``````