README.md

111. 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.

Solutions (Python)

1. Recursion

# 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: TreeNode) -> int:
        if not root:
            return 0
        if not root.left and not root.right:
            return 1
        lmin, rmin = 0, 0
        if root.left:
            lmin = self.minDepth(root.left) + 1
        if root.right:
            rmin = self.minDepth(root.right) + 1
        if lmin and rmin:
            return min(lmin, rmin)
        else:
            return lmin if lmin else rmin

2. Iteration

# 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: TreeNode) -> int:
        if not root:
            return 0
        depth = 1
        nodes = [root]
        while True:
            for node in nodes:
                if not node.left and not node.right:
                    return depth
            depth += 1
            nodes = [node.left for node in nodes if node.left] \
                + [node.right for node in nodes if node.right]