思路：
1.使用BFS，我的代码，很普通的常规写法

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    int bottom = 0;
    public int findBottomLeftValue(TreeNode root) {
        if(root == null){
            return 0;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        int size = queue.size();
        int left = 0;
        while(!queue.isEmpty()){
            boolean first = true;
            for(int i = 0;i < size;i++){
                TreeNode node = queue.poll();
                if(node != null){
                    if(first) left = node.val;
                    first = false;;
                    queue.add(node.left);
                    queue.add(node.right);
                }
            }
            size = queue.size();
        }
        return left;
    }
    
}

2.看看别人惊艳的代码吧，从右到左加入队列，这样最后一个弹出的就是最下行的最左元素了

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    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);//先加右节点，真得绝
            if(root.left != null) queue.add(root.left);
        }
        return root.val;
    }
    
}