update 001

Author: andavid

note/001/README.md

@@ -1,16 +1,50 @@
-# [xxx][title]
+# [Two Sum][title]
 
 ## Description
 
+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.
+
+**Example:**
+
+```
+Given nums = [2, 7, 11, 15], target = 9,
+
+Because nums[0] + nums[1] = 2 + 7 = 9,
+return [0, 1].
+```
 
 ## 思路
 
+给定一个数组和一个总数，返回数组中两个数的索引，这两个数的和等于给定的总数。
+
+遍历数组，使用一个 HashMap 记录数组里的数及其索引，如果总数与当前数的差值在 HashMap 里存在相应的记录，就找到了。
 
 ## [完整代码][src]
 
 ```java
-
+class Solution {
+  public int[] twoSum(int[] nums, int target) {
+    if (nums == null || nums.length < 2) {
+      return null;
+    }
+
+    Map<Integer, Integer> map = new HashMap<>();
+    for (int i = 0; i < nums.length; i++) {
+      if (map.containsKey(target - nums[i])) {
+        int[] result = new int[2];
+        result[0] = map.get(target - nums[i]);
+        result[1] = i;
+        return result;
+      }
+      map.put(nums[i], i);
+    }
+
+    return null;
+  }
+}
 ```
 
-[title]: https://leetcode.com/problems/xxx
-[src]: https://github.com/andavid/leetcode-java/blob/master/src/com/andavid/leetcode/_xxx/Solution.java
+[title]: https://leetcode.com/problems/two-sum
+[src]: https://github.com/andavid/leetcode-java/blob/master/src/com/andavid/leetcode/_001/Solution.java