HackerHeap

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]
]

Solution:
We can solve it multiple ways using Breadth First Search and Depth First Search the below solution uses Depth First Search with Back Tracking.

class HackerHeap {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
         List<List<Integer>> result = new ArrayList<List<Integer>>();
        if (root == null) {
            return result;
        }
        dfs(root, 0, result);
        return result;
    }
    public void dfs(TreeNode root, int level, List<List<Integer>> rst) {
        if (root == null) {
            return;
        }
        if (level >= rst.size()) {
            rst.add(0, new ArrayList<Integer>());
        }
        dfs(root.left, level + 1, rst);
        dfs(root.right, level + 1, rst);
        rst.get(rst.size() - level - 1).add(root.val);
    }
}