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.

难度：medium

题目：
给定二叉树，找出其最左边的结点。

注意：
你可以认为树一定不为ＮＵＬＬ

思路：
ＢＦＳ， 层次遍历（先入右子树，再入左子树）。

Runtime: 4 ms, faster than 71.59% of Java online submissions for Find Bottom Left Tree Value.

/**
 * 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);
        
        TreeNode node = root;
        while (!queue.isEmpty()) {
            node = queue.poll();
            if (node.right != null) {
                queue.add(node.right);
            }
            
            if (node.left != null) {
                queue.add(node.left);
            }
        }
        
        return node.val;
    }
}