/**

 * Definition for a binary tree node.

 * struct TreeNode {

 *     int val;

 *     TreeNode *left;

 *     TreeNode *right;

 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}

 * };

 */

class Solution {

public:

    int widthOfBinaryTree(TreeNode* root) {

        int left = , right = , res = ;

        queue<pair<int,TreeNode*>> q;

        if (root != NULL)   q.push({, root});

        while (!q.empty()) {

            int qsz = q.size();

            for (int i = ; i < qsz; i++) {

                int idx = q.front().first;

                TreeNode* cur = q.front().second;

                q.pop();

                if (i == )     left = idx;

                if (i == qsz - )   right = idx;

                if (cur->left)  q.push( { idx *, cur->left });

                if (cur->right) q.push( { idx * + , cur->right });

            }

            res = max(res, right-left+);

        }

        return res;

    }

};