lintcode70- Binary Tree Level Order Traversal II- medium

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).

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]
]

1. 和I一模一样，只是每次给result加的时候加在开头。写法是index写前面：result.add(0, level);

2.和I一模一样，只是最后加一个List逆序。Collections.reverse(result);

3. Stack 套queue，循环一次加进stack，再取出来放进result。

1的实现：

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */


public class Solution {
    /*
     * @param root: A tree
     * @return: buttom-up level order a list of lists of integer
     */
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        // write your code here
        List<List<Integer>> result = new LinkedList<List<Integer>>();
        if (root == null) {
            return result;
        }
        
        Queue<TreeNode> queue= new LinkedList<TreeNode>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            List<Integer> level = new ArrayList<Integer>();
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                level.add(node.val);
                if (node.left != null) {
                    queue.offer(node.left);
                }
                if (node.right != null) {
                    queue.offer(node.right);
                }
            }
            result.add(0, level);
        }
        return result;
    }
}

2的实现：

public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: buttom-up level order a list of lists of integer
     */
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> result = new ArrayList<>();
        if (root == null) {
            return result;
        }
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.offer(root);
        
        while (!queue.isEmpty()) {
            int size = queue.size();
            List<Integer> level = new ArrayList<>();
            for (int i = 0; i < size; i++) {
                TreeNode head = queue.poll();
                level.add(head.val);
                if (head.left != null) {
                    queue.offer(head.left);
                }
                if (head.right != null) {
                    queue.offer(head.right);
                }
            }
            result.add(level);
        }
        
        Collections.reverse(result);
        return result;
    }
}

3的实现：

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */


public class Solution {
    /*
     * @param root: A tree
     * @return: buttom-up level order a list of lists of integer
     */
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        // write your code here
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        if (root == null) {
            return result;
        }
        
        Stack<Queue<TreeNode>> stack = new Stack<Queue<TreeNode>>();
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.add(root);
        while (queue.size() > 0) {
            Queue<TreeNode> newQueue = new LinkedList<TreeNode>();
            for (TreeNode node : queue) {
                if (node.left != null) {
                    newQueue.offer(node.left);
                }
                if (node.right != null) {
                    newQueue.offer(node.right);
                }
            }
            stack.push(queue);
            queue = newQueue;
        }
        
        while (!stack.isEmpty()) {
            queue = stack.pop();
            List<Integer> level = new ArrayList<Integer>();
            for (TreeNode node : queue) {
                level.add(node.val);
            }
            result.add(level);
        }
        return result;
    }
}