leetcode111-python 二叉树的最小深度

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:

Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its minimum depth = 2.

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解题要点：

1.与求二叉树的最大深度类似，但是若有一支没有叶节点，则最小节点要从另一支开始算；

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

class Solution:
    def minDepth(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if root == None:
            return 0
        elif root.left == None and root.right == None:
            return 1
        elif root.left == None and root.right != None:
            return self.minDepth(root.right)+1
        elif root.right ==None and root.left != None:
            return self.minDepth(root.left)+1
        elif root.left != None and root.right !=None:
            return min(self.minDepth(root.left), self.minDepth(root.right))+1