Add Java codes, and license.

codes/java/chapter_computational_complexity/space_time_tradeoff/leetcode_two_sum.java

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+package chapter_computational_complexity.space_time_tradeoff;
+
+import java.util.*;
+
+class solution_brute_force {
+    public int[] twoSum(int[] nums, int target) {
+        int size = nums.length;
+        for (int i = 0; i < size - 1; i++) {
+            for (int j = i + 1; j < size; j++) {
+                if (nums[i] + nums[j] == target)
+                    return new int[] { i, j };
+            }
+        }
+        return new int[0];
+    }
+}
+
+class solution_hash_map {
+    public int[] twoSum(int[] nums, int target) {
+        int size = nums.length;
+        Map<Integer, Integer> dic = new HashMap<>();
+        for (int i = 0; i < size; i++) {
+            if (dic.containsKey(target - nums[i])) {
+                return new int[] { dic.get(target - nums[i]), i };
+            }
+            dic.put(nums[i], i);
+        }
+        return new int[0];
+    }
+}
+
+public class leetcode_two_sum {
+    public static void main(String[] args) {
+        // ======= Test Case =======
+        int[] nums = { 2,7,11,15 };
+        int target = 9;
+        
+        // ====== Driver Code ======
+        solution_brute_force slt1 = new solution_brute_force();
+        int[] res = slt1.twoSum(nums, target);
+        System.out.println(Arrays.toString(res));
+
+        solution_hash_map slt2 = new solution_hash_map();
+        res = slt2.twoSum(nums, target);
+        System.out.println(Arrays.toString(res));
+    }
+}