用 BFS，层次遍历

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public int findBottomLeftValue(TreeNode root) {        if (root == null) {            return -1;        }        Queue
     
       queue = new LinkedList<>();        queue.offer(root);        int size = 1;        int i = 0;        int result = root.val;        while (!queue.isEmpty()) {            TreeNode current = queue.poll();            if (i == 0) {                result = current.val;            }            if (current.left != null) {                queue.offer(current.left);            }            if (current.right != null) {                queue.offer(current.right);            }            i++;                        if (i == size) {                i = 0;                size = queue.size();            }        }                return result;    }}