leetcode111 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.
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.
"""
"""
解法一：迭代，用一个(node,depth) 与树结构绑定 并持续更新
"""
class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

class Solution1:
    def minDepth(self, root):
        if not root:
            return 0
        queue = []
        queue.append((root, 1)) #!!!关键记录深度
        res = float('inf') #结果初始化为最大
        while queue:
            node, depth = queue.pop(0)
            if not node.left and not node.right:
                res = min(res, depth)
            if node.left:
                queue.append((node.left, depth+1))
            if node.right:
                queue.append((node.right, depth+1))
        return res
"""
解法二：递归
"""
class Solution2:
    def minDepth(self, root):
        if not root:
            return 0
        if not root.left:
            return self.minDepth(root.right)+1
        if not root.right:
            return self.minDepth(root.left)+1
        return min(self.minDepth(root.left), self.minDepth(root.right))+1