1. function height(node){
    2.     if(!node) return 0
    3.     return (Math.max(height(node.left),height(node.right))+1);
    4. }
    1. function TreeNode(val, left, right) {
    2.     this.val = (val===undefined ? 0 : val)
    3.     this.left = (left===undefined ? null : left)
    4.    this.right = (right===undefined ? null : right)
    5. }
    7. let a = new TreeNode(2)
    8. let b = new TreeNode(1,a)
    9. let c = new TreeNode(4,a,b)
    10. let d = new TreeNode(2,c)