Solution for Two Sum #219

Author: dohyeon2

two-sum/dohyeon2.java

@@ -0,0 +1,36 @@
+import java.util.HashMap;
+
+class Solution {
+    public int[] twoSum(int[] nums, int target) {
+        // Approach : using HashMap to get index with the element in O(n) time complexity
+        // SpaceComplexity is also O(n)
+        HashMap<Integer,Integer> numIndexMap = new HashMap<Integer,Integer>();
+
+        // Make key and value HashMap
+        for(int i = 0; i < nums.length; i++){
+            int num = nums[i];
+            numIndexMap.put(num,i);
+        }
+
+        // Search for the other operand looping nums
+        for(int i = 0; i < nums.length; i++){
+            int num = nums[i];
+            int operand = target - num;
+            Integer index = numIndexMap.get(operand);
+            boolean indexExists = index != null;
+            boolean indexExistsAndIndexIsNotTheNum = indexExists && i != index;
+            if(indexExistsAndIndexIsNotTheNum){
+                // Manual sort
+                if( i < index){
+                    return new int[]{ i, index };
+                }else{
+                    return new int[]{ index, i };
+                }
+            }
+        }
+
+        // If the valid answer is always exists this is not needed
+        // But the compiler don't know about that
+        return new int[2];
+    }
+}