Problem

Given a binary tree, return the preorder traversal of its nodes’ values.

Example:

Input: [1,null,2,3]

     1
      \
       2
      /
     3

Output: [1,2,3]

Follow up: Recursive solution is trivial, could you do it iteratively?

Pre analysis

Solved recursively.

Another solution

Iterative approach

var preorderTraversal = function(root) {

  let stack = [root]
  let result = [];

  while(stack.length){
     let t = stack.pop();
     result.push(t.val);
  if(t.right){
     stack.push(t.right)
  }
  if(t.left){
     stack.push(t.left)
  }

  }


  return result
  };