Binary Tree Postorder Traversal

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

For example:
Given binary tree [1,null,2,3]

1
 \
  2
 /
3

return [3,2,1]

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */

function dfs(root, ans) {
  if (!root) return;

  dfs(root.left, ans);
  dfs(root.right, ans);

  ans.push(root.val);
}

var postorderTraversal = function(root) {
  var ans = [];
  dfs(root, ans);
  return ans;
};

Binary Tree Preorder Traversal

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

For example:
Given binary tree [1,null,2,3]

1
 \
  2
 /
3

return [1,2,3]

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
function dfs(root, ans) {
  if (!root) return;
  ans.push(root.val);

  dfs(root.left, ans);
  dfs(root.right, ans);
}

var preorderTraversal = function(root) {
  var ans = [];
  dfs(root, ans);
  return ans;
};

Binary Tree Right Side View

Given a binary tree, imagine yourself standing on the right side of it, return the values of the nodes you can see ordered from top to bottom.

For example:
Given the following binary tree,

   1            <---
 /   \
2     3         <---
 \     \
  5     4       <---

You should return [1, 3, 4]

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var rightSideView = function(root) {
  if (!root) return [];

  var ans = []
    , tmp = [root];

  while (tmp.length) {
    var res = []
      , a = [];

    for (var i = 0, len = tmp.length; i < len; i++) {
      if (!tmp[i]) continue;
      res.push(tmp[i].val);
      a.push(tmp[i].left);
      a.push(tmp[i].right);
    }

    tmp = a.concat();
    if (res.length)
      ans.push(res);
  }

  var a = [];
  ans.forEach(function(item) {
    a.push(item.pop());
  });

  return a;
};