feat : two-sum

Author: ekgns33

two-sum/ekgns33.java

@@ -0,0 +1,39 @@
+import java.util.HashMap;
+import java.util.Map;
+
+/*
+input : array of integers, single integer target
+output : indices of the two numbers that they add up to target
+constraint:
+1) is integer positive? 
+no [-10^9, 10^9]
+2) is there any duplicates?
+yes. but only one valid answer exists
+3) can i reuse elem?
+no
+
+sol1) brute force
+nested for loop. tc: O(n^2), sc: O(1) when n is the length of input
+
+sol2) better solution with hash map
+iterate through the array
+  check if target - current elem exists
+    if return pair of indices
+    else save current elem and continue
+
+tc : O(n), sc: O(n) when n is the length of input
+
+ */
+class Solution {
+  public int[] twoSum(int[] nums, int target) {
+    Map<Integer, Integer> prev = new HashMap<>();
+    for(int i = 0; i < nums.length; i++) {
+      int key = target - nums[i];
+      if(prev.containsKey(key)) {
+        return new int[] {prev.get(key), i};
+      }
+      prev.put(nums[i], i);
+    }
+    return null;
+  }
+}