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Sep 18th, 2019
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  1. //Code assembles a Binary Tree using depth first traversal, taking in the inorder and postorder of the tree to create it
  2. //Definition of a Tree Node
  3. //function TreeNode(val) {
  4. // this.val = val;
  5. // this.right = this.left = null;
  6. //}
  7.  
  8. const buildTree = function(inorder, postorder) {
  9. let hashmap = [];
  10. let post_index = postorder.length - 1;
  11. let in_left = 0;
  12. let in_right = postorder.length - 1;
  13. for(let val in inorder) {
  14. hashmap.push(inorder[val]);
  15.  
  16. return helper(in_left, in_right);
  17.  
  18. function helper(in_left, in_right) {
  19. if(in_left > in_right) return null;
  20.  
  21. //Find the value of the node, create the new node
  22. let root_val = postorder[post_index];
  23. let root = new TreeNode(root_val);
  24.  
  25. let index = hashmap.indexOf(root_val);
  26.  
  27. //Recursively use the helper function to add nodes to the tree.
  28. //Base case above will return null because the length of the array won't exist
  29. post_index--;
  30. //right first because of post order
  31. root.right = helper(index + 1, in_right);
  32. root.left = helper(in_left, index - 1);
  33. //Root returns the first root so we can observe the tree
  34. return root;
  35. }
  36.  
  37. }
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