[leetcode] Minimum Depth of Binary Tree ,到叶子节点的最小距离 （python）

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.

class Solution:
    # @param root, a tree node
    # @return an integer
    minDep =-1
    def minDepth(self, root):
        if root is None:
            return 0
        self.helper(root,1)
        return self.minDep

    # @param dep, 当前为第几层
    def helper(self,root,dep):
        if root.left is None and root.right is None:
            if self.minDep is -1:
                self.minDep =dep
            elif self.minDep >dep:
                self.minDep =dep

        if dep >= self.minDep and self.minDep is not -1:
            return False

        if root.left is not None:
            self.helper(root.left,dep+1)

        if root.right is not None:
            self.helper(root.right,dep+1)

这题我使用的是DFS，但是我设置了一个minDep的global变量。这个变量储存目前最小的【叶子节点到根的距离】，所以其他分支遍历的时候，检查一下dep（当前距离）是否大于minDep，如果大于或者等于，就结束该分支的遍历。