added 0110_balanced_binary_tree.py

Author: nyierr

0110_balanced_binary_tree.py

@@ -0,0 +1,54 @@
+"""
+Given a binary tree, determine if it is height-balanced.
+
+For this problem, a height-balanced binary tree is defined as:
+
+a binary tree in which the left and right subtrees of every node
+differ in height by no more than 1.
+
+Example 1:
+  Input: root = [3,9,20,null,null,15,7]
+  Output: true
+    
+Example 2:
+  Input: root = [1,2,2,3,3,null,null,4,4]
+  Output: false
+
+Example 3:
+  Input: root = []
+  Output: true
+
+Constraints:
+  * The number of nodes in the tree is in the range [0, 5000]
+  * -10^4 <= Node.val <= 10^4
+"""
+
+class Solution:
+    # O(2^n) solution, recursive
+    def isBalanced(self, root: Optional[TreeNode]) -> bool:
+        if root is None:
+            return True
+        
+        left = {}
+        right = {}
+        self.helper(root, left, right)
+        
+        for k in left:
+            if abs(left[k] - right[k]) > 1:
+                return False
+            
+        return True
+        
+    def helper(self, node, l={}, r={}):
+        if not node is None:
+            l[node] = 0
+            r[node] = 0
+            
+            if not node.left is None:
+                self.helper(node.left, l, r)
+                l[node] = max(l[node.left], r[node.left]) + 1
+                
+            if not node.right is None:
+                self.helper(node.right, l, r)
+                r[node] = max(l[node.right], r[node.right]) + 1
+