[20200704] Solve july challenge questions

Author: iamminji

challenge/montly/july/Binary_Tree_Level_Order_Traversal_II.md

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+## 문제
+Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).
+
+```
+For example:
+Given binary tree [3,9,20,null,null,15,7],
+    3
+   / \
+  9  20
+    /  \
+   15   7
+return its bottom-up level order traversal as:
+[
+  [15,7],
+  [9,20],
+  [3]
+]
+```
+
+### 솔루션
+알고리즘 보다는 구현 문제였다. 
+
+#### 해결
+tree 순회하면서 리스트에 값을 넣는데, 이 때 depth 를 리스트의 인덱스로 잡고 넣어주면 된다.
+리턴할 때는 리스트의 역순으로 리턴하면 정답이 나온다.
+
+```python3
+class Solution:
+    def levelOrderBottom(self, root: TreeNode) -> List[List[int]]:
+
+        if root is None:
+            return []
+
+        queue = deque()
+        queue.append([root, 0])
+
+        result = []
+
+        while queue:
+            node, depth = queue.popleft()
+
+            if len(result) <= depth:
+                result.append([node.val])
+            else:
+                result[depth].append(node.val)
+
+            if node.left is not None:
+                queue.append([node.left, depth+1])
+
+            if node.right is not None:
+                queue.append([node.right, depth+1])
+
+        return result[::-1]
+```

challenge/montly/july/Binary_Tree_Level_Order_Traversal_II.py

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+from typing import List
+from collections import deque
+
+
+# Definition for a binary tree node.
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+    def __str__(self):
+        return "TreeNode(val:%s)" % self.val
+
+
+class Solution:
+    def levelOrderBottom(self, root: TreeNode) -> List[List[int]]:
+
+        if root is None:
+            return []
+
+        queue = deque()
+        queue.append([root, 0])
+
+        result = []
+
+        while queue:
+            node, depth = queue.popleft()
+
+            if len(result) <= depth:
+                result.append([node.val])
+            else:
+                result[depth].append(node.val)
+
+            if node.left is not None:
+                queue.append([node.left, depth+1])
+
+            if node.right is not None:
+                queue.append([node.right, depth+1])
+
+        return result[::-1]
+
+
+if __name__ == '__main__':
+    # [3,9,20,null,null,15,7]
+    node = TreeNode(3)
+    node.left = TreeNode(9)
+    node.right = TreeNode(20)
+    node.right.left = TreeNode(15)
+    node.right.right = TreeNode(7)
+
+    sol = Solution()
+    print(sol.levelOrderBottom(node))