update

Author: jasoncao11

77. 平衡二叉树.md

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+***输入一棵二叉树的根节点，判断该树是不是平衡二叉树。如果某二叉树中任意节点的左右子树的深度相差不超过1，那么它就是一棵平衡二叉树。***
+
+```
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, x):
+#         self.val = x
+#         self.left = None
+#         self.right = None
+
+class Solution:
+    def isBalanced(self, root: TreeNode) -> bool:
+        #构建递归，如果是平衡二叉树则返回树的深度否则返回-1
+        def recur(root):
+            if not root:
+                return 0
+            left = recur(root.left)
+            if left == -1: 
+                return -1
+            right = recur(root.right)
+            if right == -1:
+                return -1
+            return max(left, right)+1 if abs(left-right) <=1 else -1
+    
+        return recur(root) != -1
+```

78. 二叉树展开为链表.md

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+![algo29](./images/algo29.jpg)
+![algo30](./images/algo30.jpg)
+
+```
+# Definition for a binary tree node.
+# class TreeNode(object):
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution(object):
+    def flatten(self, root):
+        """
+        :type root: TreeNode
+        :rtype: None Do not return anything, modify root in-place instead.
+        """
+
+        if not root:
+            return
+        self.flatten(root.left)
+        self.flatten(root.right)
+        tmp = root.right
+        root.right = root.left
+        root.left = None
+        while root.right:
+            root = root.right
+        root.right = tmp
+```

79. 合并二叉树.md

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+![algo31](./images/algo31.jpg)
+
+```
+# Definition for a binary tree node.
+# class TreeNode(object):
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution(object):
+    def mergeTrees(self, root1, root2):
+        """
+        :type root1: TreeNode
+        :type root2: TreeNode
+        :rtype: TreeNode
+        """
+        if not root1:
+            return root2
+
+        if not root2:
+            return root1
+
+        merged = TreeNode(root1.val+root2.val)
+        merged.left = self.mergeTrees(root1.left, root2.left)
+        merged.right = self.mergeTrees(root1.right, root2.right)
+        return merged 
+```

80. 翻转二叉树.md

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+![algo32](./images/algo32.jpg)
+
+```
+# 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
+class Solution:
+    def invertTree(self, root: Optional[TreeNode]) -> Optional[TreeNode]:
+        if not root:
+            return None
+        left = self.invertTree(root.left)
+        right = self.invertTree(root.right)
+        root.right = left
+        root.left = right
+        return root
+```