// calculate the sum of leaves node in binary tree 

const sumOfLeaf = (root) => {

  let sum = 0;

  let queue = [root];

  while (queue.length) {

    let current = queue.pop();

    if (current.left) {

      queue.push(current.left);

    }

    if (current.right) {

      queue.push(current.right);

    } else if (!current.left && !current.right) {

      sum += current.data;

    }

  }

  return sum;

};