O(n) time and o(n) space using BFS and Deque.

Author: 149ps

988. Smallest String Starting From Leaf/988. Smallest String Starting From Leaf.py

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+"""
+You are given the root of a binary tree where each node has a value in the range [0, 25] representing the letters 'a' to 'z'.
+
+Return the lexicographically smallest string that starts at a leaf of this tree and ends at the root.
+
+As a reminder, any shorter prefix of a string is lexicographically smaller.
+
+For example, "ab" is lexicographically smaller than "aba".
+A leaf of a node is a node that has no children.
+
+
+
+Example 1:
+
+
+Input: root = [0,1,2,3,4,3,4]
+Output: "dba"
+Example 2:
+
+
+Input: root = [25,1,3,1,3,0,2]
+Output: "adz"
+Example 3:
+
+
+Input: root = [2,2,1,null,1,0,null,0]
+Output: "abc"
+
+
+Constraints:
+
+The number of nodes in the tree is in the range [1, 8500].
+0 <= Node.val <= 25
+"""
+# 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 smallestFromLeaf(self, root: Optional[TreeNode]) -> str:
+        q,result = collections.deque(),None
+        q.append((root,chr(root.val+97)))
+        while q:
+            node,path = q.popleft()
+            if not node.left and not node.right:
+                if not result:
+                    result = path
+                else:
+                    result = min(result,path)
+            if node.left: q.append((node.left,chr(node.left.val+97)+path))
+            if node.right: q.append((node.right,chr(node.right.val+97)+path))
+        return result