Minimum Depth of Binary Tree

Question

Given 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.

Tags

• Tree
• BFS

Thought

Two approaches for BFS algorithm:

• queue
• recursion

Both of the implementations are provided below.

Code

queue

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def minDepth(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if not root:
            return 0
        queue = [(1, root)]
        while queue:
            level, ptr = queue.pop(0)
            if not ptr.left and not ptr.right:
                return level
            if ptr.left:
                queue.append((level + 1, ptr.left))
            if ptr.right:
                queue.append((level + 1, ptr.right))
        return level

recursion

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def minDepth(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if not root:
            return 0
        if root.left and root.right:
            return min(self.minDepth(root.left), self.minDepth(root.right)) + 1
        elif root.left:
            return self.minDepth(root.left) + 1
        elif root.right:
            return self.minDepth(root.right) + 1
        else:
            return 1