Leetcode 111: Minimum Depth of Binary Tree

https://leetcode.com/problems/minimum-depth-of-binary-tree/

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.

 

Code

# 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 root is None:
            return 0
            
        if root.left is None and root.right is None:
            return 1
        
        if root.left is None and root.right is not None:
            return self.minDepth(root.right) + 1
        
        if root.right is None and root.left is not None:
            return self.minDepth(root.left) + 1
        
        return min(self.minDepth(root.left), self.minDepth(root.right))+1
        

 

Idea

There are 5 conditions to consider:

1. if root is None, the current depth is zero
2. if both root’s left and right child are none, the current root is a leaf node which has depth 1
3. if only one of root.left or root.right child is none, the depth depends on the depth of the non-none child.
4. if both root.left and root.right child are not none, then the minimum depth is the min of minDepth(root.left) and minDepth(root.right) plus 1

 

Of course, you can take a few logical steps to further reduce lines of codes:

class Solution:
    # @param root, a tree node
    # @return an integer    
    def minDepth(self, root):
        if root == None:
            return 0
        if root.left==None or root.right==None:
            return self.minDepth(root.left)+self.minDepth(root.right)+1
        return min(self.minDepth(root.right),self.minDepth(root.left))+1