Solution2.java

/**
 * 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 {
    private int ans = 1;
    private List<Integer> t = new ArrayList<>();

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

    private void dfs(TreeNode root, int depth, int i) {
        if (root == null) {
            return;
        }
        if (t.size() == depth) {
            t.add(i);
        } else {
            ans = Math.max(ans, i - t.get(depth) + 1);
        }
        dfs(root.left, depth + 1, i << 1);
        dfs(root.right, depth + 1, i << 1 | 1);
    }
}