Write a function, leafList, that takes in the root of a binary tree and returns an array containing the values of all leaf nodes in left-to-right order.
test_00:
const a = new Node("a");
const b = new Node("b");
const c = new Node("c");
const d = new Node("d");
const e = new Node("e");
const f = new Node("f");
a.left = b;
a.right = c;
b.left = d;
b.right = e;
c.right = f;
// a
// / \
// b c
// / \ \
// d e f
leafList(a); // -> [ 'd', 'e', 'f' ]test_01:
const a = new Node("a");
const b = new Node("b");
const c = new Node("c");
const d = new Node("d");
const e = new Node("e");
const f = new Node("f");
const g = new Node("g");
const h = new Node("h");
a.left = b;
a.right = c;
b.left = d;
b.right = e;
c.right = f;
e.left = g;
f.right = h;
// a
// / \
// b c
// / \ \
// d e f
// / \
// g h
leafList(a); // -> [ 'd', 'g', 'h' ]test_02:
const a = new Node(5);
const b = new Node(11);
const c = new Node(54);
const d = new Node(20);
const e = new Node(15);
const f = new Node(1);
const g = new Node(3);
a.left = b;
a.right = c;
b.left = d;
b.right = e;
e.left = f;
e.right = g;
// 5
// / \
// 11 54
// / \
// 20 15
// / \
// 1 3
leafList(a); // -> [ 20, 1, 3, 54 ]test_03:
const x = new Node('x');
// x
leafList(x); // -> [ 'x' ]test_04:
leafList(null); // -> [ ]