Minimum depth of binary tree

Time: O(N); Space: O(H); easy

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.

Note:

  • A leaf is a node with no children.

Example 1:

Input: root = None

Output: 0

Example 2:

Input: root = {TreeNode} [3,9,20,None,None,15,7]

  3
 / \
9  20
  /  \
 15   7

Output: 2

Example 3:

Input: root = {TreeNode} [1,None,2,3]

Output: 3

Explanation:

1
 \
  2
 /
3

  • it will be serialized {1,#,2,3}

[1]:

class TreeNode(object):
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

[2]:

class Solution1(object):
    """
    Time: O(N)
    Space: O(H), H is height of binary tree
    """
    def minDepth(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if root is None:
            return 0

        if root.left and root.right:
            return min(self.minDepth(root.left), self.minDepth(root.right)) + 1
        else:
            return max(self.minDepth(root.left), self.minDepth(root.right)) + 1

[8]:

s = Solution1()

root = None
assert s.minDepth(root) == 0

root = TreeNode(3)
root.left = TreeNode(9)
root.right = TreeNode(20)
root.right.left = TreeNode(15)
root.right.right = TreeNode(7)
assert s.minDepth(root) == 2

root = TreeNode(1)
root.right = TreeNode(2)
root.right.left = TreeNode(3)
assert s.minDepth(root) == 3