Binary Tree Level Order Traversal II

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本文介绍了一种从叶节点到根节点的层次遍历二叉树的方法，并提供了一个具体的算法实现示例。

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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,#,#,15,7},

return its bottom-up level order traversal as:

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

Analysis: BFS.

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ArrayList<ArrayList<Integer>> levelOrderBottom(TreeNode root) {
        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
        Queue<TreeNode> frontier = new LinkedList<TreeNode>();
        
        if(root==null) return res;
        frontier.add(root);
        while(!frontier.isEmpty()) {
            Queue<TreeNode> next = new LinkedList<TreeNode>();
            ArrayList<Integer> cell = new ArrayList<Integer>();
            for(int i=frontier.size()-1; i>=0; i--) {
                TreeNode current = frontier.poll();
                if(current.left!=null) next.add(current.left);
                if(current.right!=null) next.add(current.right);
                cell.add(current.val);
            }
            res.add(0, cell);
            frontier = next;
        }

        return res;
    }
}