今日中等题：https://leetcode.cn/problems/find-bottom-left-tree-value/

终于走出舒适区开始做树相关的题目了，开始因为对概念不熟悉看完题目是懵逼的，但是再看题解的深度优先和广度优先的算法后，突然发现不过如此hhhh，用双向队列就能解决的问题。

queue和deque的区别

具体对比差异详见：https://blog.csdn.net/SeekN/article/details/114231727

贴下代码：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public int findBottomLeftValue(TreeNode root) {
        Deque<TreeNode> deque = new ArrayDeque<TreeNode>();
        deque.offer(root);
        int ans = 0;
        while (!deque.isEmpty()) {
            TreeNode cur = deque.poll();
            
            // 从右到左，确保左节点是最后一个
            if (cur.right != null) {
                deque.offer(cur.right);
            }
            if (cur.left != null) {
                deque.offer(cur.left);
            }
            ans = cur.val;
        }
        return ans;
    }
}