#513找树左下角的值

import java.util.*;

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int val) {
        this.val = val;
    }
}

class Solution {
    public int findBottomLeftValue(TreeNode root) {
        if (root == null) throw new IllegalArgumentException("root must not be null");
        Deque<TreeNode> queue = new ArrayDeque<>();
        queue.offer(root);
        int answer = root.val;
        while (!queue.isEmpty()) {
            // size 固定当前层节点数，后续入队的子节点不会混入本层扫描。
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                // 每层第一个出队节点最靠左，逐层覆盖后留下的就是最深层答案。
                if (i == 0) answer = node.val;
                if (node.left != null) queue.offer(node.left);
                if (node.right != null) queue.offer(node.right);
            }
        }
        return answer;
    }
}