add two int sum article

Author: neetcode-gh

articles/two-integer-sum.md

@@ -0,0 +1,60 @@
+## 1. Brute Force
+
+```python
+class Solution:
+    def twoSum(self, nums: List[int], target: int) -> List[int]:
+        for i in range(len(nums)):
+            for j in range(i + 1, len(nums)):
+                if nums[i] + nums[j] == target:
+                    return [i, j]
+        return []
+```
+
+### Time & Space Complexity
+
+* Time complexity: $O(n^2)$
+* Space complexity: $O(1)$
+
+---
+
+## 2. Hash Map (Two Pass)
+
+```python
+class Solution:
+    def twoSum(self, nums: List[int], target: int) -> List[int]:
+        indices = {}  # val -> index
+
+        for i, n in enumerate(nums):
+            indices[n] = i
+
+        for i, n in enumerate(nums):
+            diff = target - n
+            if diff in indices and indices[diff] != i:
+                return [i, indices[diff]]
+```
+
+### Time & Space Complexity
+
+* Time complexity: $O(n)$
+* Space complexity: $O(n)$
+
+---
+
+## 3. Hash Map (One Pass)
+
+```python
+class Solution:
+    def twoSum(self, nums: List[int], target: int) -> List[int]:
+        prevMap = {}  # val -> index
+
+        for i, n in enumerate(nums):
+            diff = target - n
+            if diff in prevMap:
+                return [prevMap[diff], i]
+            prevMap[n] = i
+```
+
+### Time & Space Complexity
+
+* Time complexity: $O(n)$
+* Space complexity: $O(n)$