111-Minimum Depth of Binary Tree

100-Same Tree | Links:

题意

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 -> 3, 9

思路

• 树：双pre，同时走两边
• root到最近的叶子节点：一共走的节点数

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution(object):
    def minDepth(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if not root: return 0  
        if root.left == None or root.right==None:  #一边为空的情况：左边为空，走右边； 右边为空，走左边 -> 取max + root自己
            return max(self.minDepth(root.left), self.minDepth(root.right)) + 1
        return min(self.minDepth(root.left), self.minDepth(root.right)) +1 # 左右都有，两边走 -> 取min + root自己

分析:

• Time: O(n)
• Space: O(n)