Create 653._Two_Sum_IV_-_Input_is_a_BST.md

Author: 0xkookoo

docs/Leetcode_Solutions/Python/653._Two_Sum_IV_-_Input_is_a_BST.md

@@ -0,0 +1,85 @@
+# 653. Two Sum IV - Input is a BST
+
+**<font color=red>难度: Easy</font>**
+
+## 刷题内容
+
+> 原题连接
+
+* https://leetcode.com/problems/two-sum-iv-input-is-a-bst/description/
+
+> 内容描述
+
+```
+Given a Binary Search Tree and a target number, return true if there exist two elements in the BST such that their sum is equal to the given target.
+
+Example 1:
+Input: 
+    5
+   / \
+  3   6
+ / \   \
+2   4   7
+
+Target = 9
+
+Output: True
+Example 2:
+Input: 
+    5
+   / \
+  3   6
+ / \   \
+2   4   7
+
+Target = 28
+
+Output: False
+```
+
+## 解题方案
+
+> 思路 1
+******- 时间复杂度: O(N)******- 空间复杂度: O(N)******
+
+
+inorder遍历是O(N),后面的二分是O(lgN)，总的时间复杂度为O(N)
+
+```python
+class Solution(object):
+    def findTarget(self, root, k):
+        """
+        :type root: TreeNode
+        :type k: int
+        :rtype: bool
+        """
+        def inorder(root):
+            if not root: 
+                return
+            inorder(root.left)
+            nums.append(root.val)
+            inorder(root.right)
+            
+        nums = []
+        inorder(root)
+        l, r = 0, len(nums) - 1
+        while l < r:
+            if nums[l] + nums[r] == k:
+                return True
+            elif nums[l] + nums[r] < k:
+                l += 1
+            else:
+                r -= 1
+        return False
+```
+
+
+
+
+
+
+
+
+
+
+