Binary Tree Level Order Traversal

Given a binary tree, return the level order traversal of its nodes’ values. (ie, from left to right, level by level).

For example:
Given binary tree [3,9,20,null,null,15,7],

  3
 / \
9  20
  /  \
 15   7

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]

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

  return ans;
};

Binary Tree Level Order Traversal II

Given a binary tree, return the bottom-up level order traversal of its nodes’ values. (ie, from left to right, level by level from leaf to root).

For example:
Given binary tree [3,9,20,null,null,15,7],

  3
 / \
9  20
  /  \
 15   7

return its bottom-up level order traversal as:

[
  [15,7],
  [9,20],
  [3]
]

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

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

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

  	tmp.forEach(function(item) {
  		res.push(item.val);
  		if (item.left)
  			_tmp.push(item.left);
  		if (item.right)
  			_tmp.push(item.right);
  	});

  	ans.push(res);
  	tmp = _tmp;
  }

  return ans.reverse();

};

Binary Tree Paths

Given a binary tree, return all root-to-leaf paths.

For example, given the following binary tree:

   1
 /   \
2     3
 \
  5

All root-to-leaf paths are:

["1->2->5", "1->3"]

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

var ans, res;

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

  res.push(root.val);

  if (!root.left && !root.right) {
    var str = res.reduce(function(pre, item) {
      return pre + '->' + item;
    });

    str = str.toString();

    ans.push(str);
    res.pop();
    return;
  }

  if (root.left) 
    dfs(root.left);
  
  if (root.right)
    dfs(root.right);

  res.pop();
}

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