513. Find Bottom Left Tree Value_python 513. find bottom left tree value (easy)-优快云博客

给定一棵二叉树，找到最后一层最左边的节点的值。方法包括直接层序遍历和利用队列实现从右到左的层序遍历。

题目描述

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

Appraoch1:
简单粗暴的方法

class Solution {
    //Runtime: 3 ms, faster than 95.43%
    //Memory Usage: 39.8 MB, less than 11.18%
    int depth, ans;
    public int findBottomLeftValue(TreeNode root) {
        depth = 0; 
        ans = root.val;
        if(root.left == null && root.right == null)
            return ans;
        find_Helper(root, 0);
        return ans; 
    }
    public void find_Helper(TreeNode root, int sub_depth){
        if(root == null) return;
        if(root.left != null && root.left.left == null && root.left.right == null){
            if((sub_depth + 1) > depth){
                depth = sub_depth + 1;
                ans = root.left.val;
            }
        }
        if(root.right != null && root.right.left == null && root.right.right == null){
            if((sub_depth + 1) > depth){
                depth = sub_depth;
                ans = root.right.val;
            }
        }
        find_Helper(root.left, sub_depth + 1);
        find_Helper(root.right, sub_depth + 1);
    }
}

Appraoch2:
利用队列的数据结构，进行从右至左的层序遍历

class Solution {
    //Runtime: 4 ms, faster than 64.10%
    //Memory Usage: 40.2 MB, less than 5.30%
    public int findBottomLeftValue(TreeNode root) {
    Queue<TreeNode> queue = new LinkedList<>();
    queue.add(root);
    while (!queue.isEmpty()) {
        root = queue.poll();
        if (root.right != null)
            queue.add(root.right);
        if (root.left != null)
            queue.add(root.left);
    }
    return root.val;
}
}