Solution2.cpp

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<int> t;
    int ans = 1;
    using ull = unsigned long long;

    int widthOfBinaryTree(TreeNode* root) {
        dfs(root, 0, 1);
        return ans;
    }

    void dfs(TreeNode* root, int depth, ull i) {
        if (!root) return;
        if (t.size() == depth) {
            t.push_back(i);
        } else {
            ans = max(ans, (int) (i - t[depth] + 1));
        }
        dfs(root->left, depth + 1, i << 1);
        dfs(root->right, depth + 1, i << 1 | 1);
    }
};