init lc repo

Author: Coder-AndyLee

Tree/102. 二叉树的层序遍历.md

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+# Description
+- 从上到下按层打印二叉树，同一层的节点按从左到右的顺序打印，每一层打印到一行
+- 例如:
+	- 给定二叉树: ```[3,9,20,null,null,15,7]```,
+```python
+
+    3
+   / \
+  9  20
+    /  \
+   15   7
+ ```
+返回其层次遍历结果：
+```python
+[
+  [3],
+  [9,20],
+  [15,7]
+]
+```
+
+# Solution
+- 层次遍历：使用栈保存每一层的节点
+- 在每一层中单独设置一个list保存
+- 注意None节点的判断
+```python
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, x):
+#         self.val = x
+#         self.left = None
+#         self.right = None
+
+class Solution:
+    def levelOrder(self, root: TreeNode) -> List[List[int]]:
+        if not root:
+            return []
+        else:
+            ans, stack = [], [root]
+            while stack:
+                layer_node, layer_val = [], []
+                while stack:
+                    current = stack.pop(0)
+                    layer_val.append(current.val)
+
+                    if current.left:
+                        layer_node.append(current.left)
+                    if current.right:
+                        layer_node.append(current.right)
+                
+                stack.extend(layer_node)
+                ans.append(layer_val)
+            
+            return ans
+        
+```

Tree/589. N叉树的前序遍历.md

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+# Description
+给定一个 N 叉树，返回其节点值的前序遍历。
+
+例如，给定一个 3叉树 :
+
+ ![[Pasted image 20200922222113.png]]
+
+
+返回其前序遍历: [1,3,5,6,2,4]。
+
+
+# Solution
+## 1. 递归
+- 基于递归方法的先序遍历
+```python
+"""
+# Definition for a Node.
+class Node:
+    def __init__(self, val=None, children=None):
+        self.val = val
+        self.children = children
+"""
+
+class Solution:
+	def VLR(self, root):
+		if root:
+			self.ans.append(root.val)
+			for i in root.children:
+				VLR(i)
+    
+	def preorder(self, root: 'Node') -> List[int]:
+        self.ans = []
+        self.VLR(root)
+        return self.ans
+```
+## 2. 循环
+- 用栈（准确地说是个list）保存待遍历节点
+- 需要注意存节点入栈的顺序
+```python
+"""
+# Definition for a Node.
+class Node:
+    def __init__(self, val=None, children=None):
+        self.val = val
+        self.children = children
+"""
+
+class Solution:
+    def preorder(self, root: 'Node') -> List[int]:
+        if not root:
+            return []
+        else:
+            ans = []
+            stack = [root]
+            while len(stack) > 0:
+                current = stack.pop(0)
+                if not current:
+                    continue
+                ans.append(current.val)
+                stack = current.children + stack
+            return ans
+```