111. Minimum Depth of Binary Tree
Easy LeetCodeGiven a binary tree, find its minimum depth.
The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.
Note: A leaf is a node with no children.
Example 1
Input: root = [3,9,20,null,null,15,7]
Output: 2
Example 2
Input: root = [2,null,3,null,4,null,5,null,6]
Output: 5
Constraints
- The number of nodes in the tree is in the range [0, 10^5].
- -1000 <= Node.val <= 1000
How to solve the problem
Recursion (DFS Only Postorder Traverse)
python
# 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 minDepth(self, root: Optional[TreeNode]) -> int:
if root is None:
return 0
if root.left is None and root.right is None:
return 1
if root.left is None:
return self.minDepth(root.right) + 1 # If left is None, go to right node
if root.right is None:
return self.minDepth(root.left) + 1
left_height = self.minDepth(root.left)
right_height = self.minDepth(root.right)
return min(left_height, right_height) + 1
Complexity
- Time complexity: O(n), n == number of nodes
- Space complexity: O(n)
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