513. 找树左下角的值

给定一个二叉树的 根节点 root，请找出该二叉树的 最底层 最左边 节点的值。

假设二叉树中至少有一个节点。

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/find-bottom-left-tree-value
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

import java.util.LinkedList;

class Solution {
    public int findBottomLeftValue(TreeNode root) {
        if (root == null) {
            return 0;
        }

        TreeNode ret = root;

        LinkedList<TreeNode> queue = new LinkedList<>();
        TreeNode curEnd = root, nextEnd = null;
        queue.offer(root);
        while (!queue.isEmpty()) {
            TreeNode node = queue.poll();
            if (node.left != null) {
                if (nextEnd == null) {
                    ret = node.left;
                }
                nextEnd = node.left;
                queue.offer(node.left);
            }

            if (node.right != null) {
                if (nextEnd == null) {
                    ret = node.right;
                }
                nextEnd = node.right;
                queue.offer(node.right);
            }

            if (node == curEnd) {
                curEnd = nextEnd;
                if (nextEnd != null) {
                    nextEnd = null;
                }
            }
        }

        return ret.val;
    }

    public static void main(String[] args) {
        TreeNode n2 = new TreeNode(2);
        TreeNode n1 = new TreeNode(1);
        TreeNode n3 = new TreeNode(3);
        n2.left = n1;
        n2.right = n3;
        System.out.println(new Solution().findBottomLeftValue(n2));
    }
}


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;
    }
}