regular leetcode

Author: KrisYu

144._binary_tree_preorder_traversal.md

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+###144. Binary Tree Preorder Traversal
+
+题目:
+
+<https://leetcode.com/problems/binary-tree-preorder-traversal/>
+
+
+难度:
+
+Medium
+
+Recursive
+
+
+```
+class Solution(object):
+    def preorderTraversal(self, root):
+        """
+        :type root: TreeNode
+        :rtype: List[int]
+        """
+        if root == None:
+            return []
+        res = []
+        self.preorder(root,res)
+        return res
+        
+        
+    def preorder(self,root,res):
+        if root == None:
+            return
+        res.append(root.val)
+        self.preorder(root.left,res)
+        self.preorder(root.right,res)
+```
+
+
+Iterative
+
+
+```
+class Solution(object):
+    def preorderTraversal(self, root):
+        """
+        :type root: TreeNode
+        :rtype: List[int]
+        """
+        if root == None:
+            return []
+        res = []
+        
+        stack = []
+        stack.append(root)
+        while stack:
+            node = stack.pop()
+            res.append(node.val)
+            if node.right:
+                stack.append(node.right)
+            if node.left:
+                stack.append(node.left)
+        return res
+
+```

168._excel_sheet_column_title.md

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+###168. Excel Sheet Column Title
+
+题目:
+
+<https://leetcode.com/problems/excel-sheet-column-title/>
+
+
+难度:
+
+Easy
+
+依旧26进制的反击，不过这个反击我做的没之前那个好，看了hint
+
+```
+class Solution(object):
+    def convertToTitle(self, n):
+        """
+        :type n: int
+        :rtype: str
+        """
+        ans = ''
+        while n :
+            ans = chr(ord('A') + (n - 1) % 26) + ans
+            n = (n - 1) // 26
+        return ans
+        
+```
+

171._excel_sheet_column_number.md

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+###171. Excel Sheet Column Number
+
+题目:
+
+<https://leetcode.com/problems/excel-sheet-column-number/>
+
+
+难度:
+
+Easy
+
+
+26进制的反击
+
+```
+class Solution(object):
+    def titleToNumber(self, s):
+        """
+        :type s: str
+        :rtype: int
+        """
+        maps = {}
+        for i in range(65,91):
+            maps[chr(i)] = i - 64
+        
+        lst = list(s)
+        lst.reverse()
+        num = 0
+        for idx,item in enumerate(lst):
+            num += maps[item] * (26 ** idx)
+        return num
+        
+```
+

199._binary_tree_right_side_view.md

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+###199. Binary Tree Right Side View
+
+题目:
+
+<https://leetcode.com/problems/binary-tree-right-side-view/>
+
+
+难度:
+
+Medium
+
+
+还是在玩第102题，level order traversal.
+
+```
+class Solution(object):
+    def rightSideView(self, root):
+        """
+        :type root: TreeNode
+        :rtype: List[int]
+        """
+        if root == None: return []
+      
+        res = []
+        curLevel = [root]
+        while curLevel:
+            nextLevel = []
+            tmpRes = []
+            for node in curLevel:
+                tmpRes.append(node.val)
+                if node.left: nextLevel.append(node.left)
+                if node.right: nextLevel.append(node.right)
+            res.append(tmpRes[-1])
+            curLevel = nextLevel
+        return res
+```
+