feat: solve two sum

Author: GangBean

two-sum/GangBean.java

@@ -0,0 +1,35 @@
+class Solution {
+    public int[] twoSum(int[] nums, int target) {
+        /**
+            1. understanding
+            - find the indices where the numbers of that index pair sum upto the target number.
+            - exactly one solution
+            - use each element only once
+            - return in any order
+            2. strategy
+            - brute force:
+                - validate for all pair sum
+            - hashtable:
+                - assing hashtable variable to save the nums and index
+                - while iterate over the nums,
+                - calculate the diff, and check if the diff in the hashtable.
+                - or save new entry.
+            3. complexity
+            - time: O(N)
+            - space: O(N)
+        */
+
+        Map<Integer, Integer> map = new HashMap<>();
+
+        for (int i = 0; i < nums.length; i++) {
+            int diff = target - nums[i];
+            if (map.containsKey(diff)) {
+                return new int[] {map.get(diff), i};
+            }
+            map.put(nums[i], i);
+        }
+
+        return new int[] {};
+    }
+}
+