Merge pull request #2581 from MengyiG/mengyiTwoSum

Update problem 1 with running map solution in Java.

problems/0001.两数之和.md

@@ -152,6 +152,24 @@ public int[] twoSum(int[] nums, int target) {
     return res;
 }
 ```
+
+```java
+//使用哈希表方法2
+public int[] twoSum(int[] nums, int target) {
+    Map<Integer, Integer> indexMap = new HashMap<>();
+    
+    for(int i = 0; i < nums.length; i++){
+        int balance = target - nums[i];  // 记录当前的目标值的余数
+        if(indexMap.containsKey(balance)){  // 查找当前的map中是否有满足要求的值
+            return new int []{i, indexMap.get(balance)}; //  如果有，返回目标值
+        } else{
+            indexMap.put(nums[i], i); //  如果没有，把访问过的元素和下标加入map中
+        }
+    }
+    return null;
+}
+```
+
 ```java
 //使用双指针
 public int[] twoSum(int[] nums, int target) {