SOURCE

function f(root) {
    let re = [];
    let stack = [];
    if(root !== null) {
        stack.push(root);
    }
    while(stack.length) {
        let node = stack.pop();
        re.push(node.val);
        if(node.right) {
            stack.push(node.right);
        }
        if(node.left) {
            stack.push(node.left);
        }
    }
    return re;
}

var root = {
val: 1,
left: {

val: 2,
left: {
  val: 4,
},
right:{
  val:5
}
},
right: {

val: 3,
left: {
  val: 6
},
right: {
  val: 7
}
}
}

console.log(f(root));