Daily Leetcode 513. Find Bottom Left Tree Value

https://leetcode.com/problems/find-bottom-left-tree-value/

Medium

问题描述：

Given a binary tree, find the leftmost value in the last row of the tree.

Example 1:

Input:

    2
   / \
  1   3

Output:
1

Example 2:

Input:

        1
       / \
      2   3
     /   / \
    4   5   6
       /
      7

Output:
7

Note: You may assume the tree (i.e., the given root node) is not NULL.

题目分析：

所谓leftmost，就是要找到给定二叉树最深层次、位置在最左边的那个节点，二叉树相关的问题基本上都可以转换成树的遍历的问题。我们可以以先序顺序遍历给定二叉树，在遍历过程中，更新最大深度和对应的最大值，只有当前节点的深度大于遍历至今保存的最大深度时，才进行上述更新操作。

代码：

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

class Solution(object):
    def findBottomLeftValue(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        self.currentMaxVal = root.val
        self.currentMaxDepth = 0
        
        def helper(root, depth):
            if not root:
                return 0
            
            if depth > self.currentMaxDepth:
                self.currentMaxVal = root.val
                self.currentMaxDepth = depth
            
            if root.left:
                helper(root.left, depth + 1)
            
            if root.right:
                helper(root.right, depth + 1)
        
        helper(root, 1)
        
        return self.currentMaxVal