二叉樹層序遍歷II

原題

　　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]
]

 

題目大意

　　給定一棵二叉樹自底向下進行層序遍歷。

解題思路

　　對樹進行層序遍歷，每層的結果放在結果鏈表的頭部。

代碼實現

 

public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x; }
}

 

算法實現類

import java.util.*;

public class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {

        List<List<Integer>> list = new LinkedList<>();
        if (root == null) {
            return list;
        }

        Deque<TreeNode> cur = new LinkedList<>();
        Deque<TreeNode> nxt = new LinkedList<>();
        Deque<TreeNode> exc = new LinkedList<>();

        TreeNode tmp;

        cur.add(root);
        while (!cur.isEmpty()) {
            List<Integer> layout = new ArrayList<>();

            while (!cur.isEmpty()) {
                tmp = cur.remove();

                if (tmp.left != null) {
                    nxt.add(tmp.left);
                }

                if (tmp.right != null) {
                    nxt.add(tmp.right);
                }

                layout.add(tmp.val);
            }

            exc = cur;
            cur = nxt;
            nxt = exc;

            list.add(0, layout);
        }

        return list;
    }
}