solve two-sum

Author: samcho0608

two-sum/samcho0608.java

@@ -0,0 +1,27 @@
+import java.util.HashMap;
+import java.util.Map;
+
+class Solution {
+    // return: indices of two numbers that add up to `target`
+    // * exactly one solution
+    // * must use index only once
+
+    // Time Complexity: O(n)
+    // Space Complexity: O(n)
+    public int[] twoSum(int[] nums, int target) {
+        Map<Integer, Integer> indexByNum = new HashMap<>();
+
+        for(int i = 0; i < nums.length; i++) {
+            int numI = nums[i];
+            indexByNum.put(numI, i);
+        }
+
+        for(int i = 0; i < nums.length; i++) {
+            int numI = nums[i];
+            Integer compl = indexByNum.getOrDefault(target-numI, null);
+            if(compl != null && i != compl) return new int[] {i, compl};
+        }
+
+        return null;
+    }
+}