107. Binary Tree Level Order Traversal II

最新推荐文章于 2021-08-10 13:02:56 发布

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最新推荐文章于 2021-08-10 13:02:56 发布

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原文链接：http://www.cnblogs.com/ycled/p/3334145.html

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，并提供了详细的算法实现步骤。通过递归的方式记录每一层的节点值，最后反转结果列表得到自底向上的层次遍历。

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],
]

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

---

同basic travel by level, 只不过最后把arraylist reverse一下

---

/**
 * 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>> rst = new ArrayList<ArrayList<Integer>>();     
        helper(root, 0, rst);
        //reverse the arraylist
        Collections.reverse(rst);
        return rst;
        
    }
    
    
    private void helper(TreeNode node,  int level, 
            ArrayList<ArrayList<Integer>> rst){
        
        if(node == null) return;
        
        ArrayList<Integer> list = null;
        
        if(rst.size() == level){ // a new level
            list = new ArrayList<Integer>();
            list.add(node.val);
            rst.add(list);
            
        }else{
            
            list = rst.get(level);
            list.add(node.val);
        }
        
        helper(node.left, level+1, rst);
        helper(node.right, level+1, rst);
    }
}