binaryTreeLevelOrderTraversalII.java

/*
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]
]
*/
public class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        Queue<TreeNode> queue = new LinkedList<>();
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        if(root == null) return result;

        queue.offer(root);
        while(!queue.isEmpty()){
            List<Integer> list = new ArrayList<>();
            int treeLevel = queue.size();       //element in each level
            for(int i = 0; i < treeLevel; i++){
                if(queue.peek().left != null) queue.offer(queue.peek().left);
                if(queue.peek().right != null) queue.offer(queue.peek().right);
                list.add(queue.poll().val);
            }
            result.add(0, list);
        }
        return result;
    }
}