Number 1

Author: Alex-Zhou99

1. Two Sum.java

@@ -0,0 +1,42 @@
+public class TwoSum {
+
+    /**
+     *  1. Two Sum
+
+     Given an array of integers, return indices of the two numbers such that they add up to a specific target.
+
+     You may assume that each input would have exactly one solution, and you may not use the same element twice.
+
+     Given nums = [2, 7, 11, 15], target = 9,
+
+     Because nums[0] + nums[1] = 2 + 7 = 9,
+
+     return [0, 1].
+
+     time : O(n)
+     space : O(n)
+     * @param nums
+     * @param target
+     * @return
+     */
+
+    public static int[] twoSum(int[] nums, int target) {
+
+       if (nums == null || nums.length < 2) {
+           return new int[]{-1, -1};
+       }
+
+       int[] res = new int[]{-1, -1};
+       HashMap<Integer, Integer> map = new HashMap<>();
+       for (int i = 0; i < nums.length; i++) {
+           if (map.containsKey(target - nums[i])) {
+               res[0] = map.get(target - nums[i]);
+               res[1] = i;
+               break;
+           }
+           map.put(nums[i], i);
+       }
+
+       return res;
+    }
+}