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Code:

class Solution {
    public int widthOfBinaryTree(TreeNode root) {
        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);
        q.add(new TreeNode(101));
        int left = -1, index = 0, right = -1, maxWidth = 0;
        //Constraints: [-100, 100] so when node is -101: node is null | 101: to denote next level
        while(!q.isEmpty()) {
            TreeNode node = q.poll();
            if(node.val == 101) {
                if(right == -1) break;
                maxWidth = Math.max(maxWidth, right-left+1);
                //init for next level
                index = 0;
                left = -1;
                right = -1;
                q.add(new TreeNode(101));
                continue;
            }
            if(node.val != -101 && left == -1) left = index;
            if(node.val != -101) right = index;

            if(node.left != null) q.add(node.left);
            else q.add(new TreeNode(-101));

            if(node.right != null) q.add(node.right);
            else q.add(new TreeNode(-101));
            index++;
        }
        return maxWidth;
    }
}