feat: two-sum

Author: minji-go

two-sum/minji-go.java

@@ -1,21 +1,24 @@
-/*
-    Problem: https://leetcode.com/problems/two-sum/
-    Description: return indices of the two numbers such that they add up to target. not use the same element twice.
-    Topics: Array, Hash Table
-    Time Complexity: O(N), Runtime 2ms
-    Space Complexity: O(N), Memory 45.1MB
-*/
+/**
+ * <a href="https://leetcode.com/problems/two-sum/">week01-2.two-sum</a>
+ * <li> Description: return indices of the two numbers such that they add up to target. not use the same element twice. </li>
+ * <li> Topics: Array, Hash Table             </li>
+ * <li> Time Complexity: O(N), Runtime 2ms    </li>
+ * <li> Space Complexity: O(N), Memory 45.4MB </li>
+ */
+
 class Solution {
     public int[] twoSum(int[] nums, int target) {
-        Map<Integer, Integer> numIndex = new HashMap<>();
-        for(int secondIndex=0; secondIndex<nums.length; secondIndex++){
-            if(numIndex.containsKey(target-nums[secondIndex])){
-                int firstIndex = numIndex.get(target-nums[secondIndex]);
-                return new int[]{firstIndex, secondIndex};
-            } else {
-                numIndex.put(nums[secondIndex], secondIndex);
-            }
+        Map<Integer, Integer> seen = new HashMap<>();
+
+        for (int i = 0; i < nums.length; i++) {
+            int num = target - nums[i];
+
+            if (seen.containsKey(num))
+                return new int[]{seen.get(num), i};
+
+            seen.put(nums[i], i);
         }
-        return new int[]{};
+
+        throw new IllegalArgumentException();
     }
 }