107_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,#,#,15,7},
 *     3
 *    / \
 *   9  20
 *     /  \
 *    15   7
 * return its bottom-up level order traversal as:
 * [
 *   [15,7]
 *   [9,20],
 *   [3],
 * ]
 * /
 /**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

 //直接翻转就可以了
 public class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        List<TreeNode> cur = new ArrayList<TreeNode>();
        if(root != null) cur.add(root);
        while(!cur.isEmpty()) {
            List<Integer> list = new ArrayList<Integer>();
            List<TreeNode> next = new ArrayList<TreeNode>();
            for(TreeNode node : cur) {
                list.add(node.val);
                if(node.left != null) next.add(node.left);  
                if(node.right != null) next.add(node.right);
            }
            res.add(list);
            cur = next;
        }
        Collections.reverse(res);
        return res;
    }
}
 //same as I
public class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        List<TreeNode> cur = new ArrayList<TreeNode>();
        if(root == null) return res;
        cur.add(root);
        traverse(res, cur);
        return res;
    }
    private void traverse(List<List<Integer>> res, List<TreeNode> cur) {
        List<Integer> list = new ArrayList<Integer>();
        List<TreeNode> next = new ArrayList<TreeNode>();
        for(TreeNode node : cur) {
            list.add(node.val);
            if(node.left != null) next.add(node.left);
            if(node.right != null) next.add(node.right);
        }
        if(!next.isEmpty()) traverse(res, next);
        res.add(list);
    }
}

//Queue
public class BinaryTreeLevelOrderTraversalII {
 
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        Queue<TreeNode> cur = new LinkedList<TreeNode>();
        if (root != null)
            cur.offer(root);
        Queue<TreeNode> next = new LinkedList<TreeNode>();
        List<Integer> list = new ArrayList<Integer>();
        while (!cur.isEmpty()) {
            TreeNode node = cur.poll();
            list.add(node.val);
            if (node.left != null)
                next.offer(node.left);
            if (node.right != null)
                next.offer(node.right);
            if (cur.isEmpty()) {
                result.add(list);
                list = new ArrayList<Integer>();
                cur = next;
                next = new LinkedList<TreeNode>();
            }
        }
        Collections.reverse(result);
        return result;
    }

}