维护两个队列, 一个是node, 一个是这个node在一个完全二叉树中的位置, 每层记录最左边的节点和最右边的节点的位置, 计算ans

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;
    }
}
// @lc code=start
/**
 * Definition for a binary tree node.
 * public 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;
 *     }
 * }
 */
class Solution {
    /**
     * 维护两个队列, 一个是node, 一个是这个node在一个完全二叉树中的位置,
     * 每层记录最左边的节点和最右边的节点的位置, 计算ans
     */
    public int widthOfBinaryTree(TreeNode root) {
        if (root.left == null && root.right == null) return 1;
        int ans = 0;
        Deque<TreeNode> que = new ArrayDeque<>();
        Deque<Integer> pos = new ArrayDeque<>();
        que.addLast(root);
        pos.addLast(1);
        while (!que.isEmpty()) {
            int size = que.size();
            int left = -1;
            int right = -1;
            for (int i = 0; i < size; i++) {
                TreeNode node = que.removeFirst();
                int p = pos.removeFirst();
                if (node.left != null) {
                    que.add(node.left);
                    pos.add(p*2+1);
                }
                if (node.right != null) {
                    que.add(node.right);
                    pos.add(p*2+2);
                }
                if (left == -1) left = p;
                right = p;

            }
            ans = Math.max(ans, right - left + 1);
        }
        return ans;
    }
}