0513找树左下角的值 Marathon

给定一个二叉树的 根节点 root，请找出该二叉树的 最底层 最左边 节点的值。

假设二叉树中至少有一个节点。

示例 1:

输入: root = [2,1,3]
输出: 1
示例 2:

输入: [1,2,3,4,null,5,6,null,null,7]
输出: 7

提示:

二叉树的节点个数的范围是 [1,104]
-231 <= Node.val <= 231 - 1

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/find-bottom-left-tree-value

• https://leetcode-cn.com/problems/find-bottom-left-tree-value/solution/dai-ma-sui-xiang-lu-dai-ni-xue-tou-er-ch-w3og/

python

# 513.找树左下角的值
class Solution:
    def findBottomLeftValue(self, root: TreeNode) -> int:
        """
        前序递归
        :param root:
        :return:
        """
        max_depth = -float("INF")
        leftMostVal = 0
        def traverse(root, curDepth):
            nonlocal max_depth, leftMostVal
            # 终止条件
            if not root.left and not root.right:
                if curDepth > max_depth:
                    max_depth = curDepth
                    leftMostVal = root.val
            if root.left:
                curDepth += 1
                traverse(root.left, curDepth)
                curDepth -= 1
            if root.right:
                curDepth += 1
                traverse(root.right, curDepth)
                curDepth -= 1
        traverse(root, 0)
        return leftMostVal

class Solution2:
    def findBottomLeftValue(self, root: TreeNode) -> int:
        """
        迭代层序遍历
        :param root:
        :return:
        """
        from collections import deque
        queue = deque()
        if root:
            queue.append(root)
        res = 0
        while queue:
            queueLen = len(queue)
            for i in range(queueLen):
                if i == 0:
                    res = queue[i].val
                cur = queue.popleft()
                if cur.left:
                    queue.append(cur.left)
                if cur.right:
                    queue.append(cur.right)
        return res

golang

package binaryTree

import "container/list"

var maxDepth int
var value int

func findBottomLeftValue(root *TreeNode) int {
	if root.Left == nil && root.Right == nil {
		return root.Val
	}
	findLeftValue(root, maxDepth)
	return value
}

func findLeftValue(root *TreeNode, deep int)  {
	// 最左边的值在左边
	if root.Left == nil && root.Right == nil {
		if deep > maxDepth {
			value = root.Val
			maxDepth = deep
		}
	}
	// 递归左树
	if root.Left != nil {
		deep++
		findLeftValue(root.Left, deep)
		deep--
	}
	// 递归右树
	if root.Right != nil {
		deep++
		findLeftValue(root.Right, deep)
		deep--
	}
}


// 迭代法
func findBottomLeftValue2(root *TreeNode) int {
	queue := list.New()
	var res int
	queue.PushBack(root)
	for queue.Len() > 0 {
		length := queue.Len()
		for i:=0;i<length;i++ {
			node := queue.Remove(queue.Front()).(*TreeNode)
			if i == 0 {
				res = node.Val
			}
			if node.Left != nil {
				queue.PushBack(node.Left)
			}
			if node.Right != nil {
				queue.PushBack(node.Right)
			}
		}
	}
	return res
}