前序遍历（先根次序遍历）
    先打印当前的，再打印左边的，再打印右边的。
    如下代码

    1. class Node {
    2.     constructor(value) {
    3.         this.value = value;
    4.         this.left - null;
    5.         this.right = null;
    6.     }
    7. }
    9. let a = new Node("A")
    10. let b = new Node("B")
    11. let c = new Node("C")
    12. let d = new Node("D")
    13. let e = new Node("E")
    14. let f = new Node("F")
    15. let g = new Node("G")
    17. a.left = c;
    18. a.right = b;
    19. c.left = f;
    20. c.right = g;
    21. b.left = d;
    22. b.right = e
    24. function fl(root){
    25.     if(root == null) return;
    26.     console.log(root.value)
    27.     fl(root.left)
    28.     fl(root.right)
    29. }
    30. fl(a)