二叉树的层序遍历 II
题目介绍
给定一个二叉树，返回其节点值自底向上的层序遍历。 （即按从叶子节点所在层到根节点所在的层，逐层从左向右遍历）
例如：
给定二叉树 [3,9,20,null,null,15,7],
返回其自底向上的层序遍历为：
题目解法
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| package algorithm;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;
public class BinaryTreeLevelOrderTraversalII {
public static List<List<Integer>> levelOrderBottom(TreeNode root) {
List<List<Integer>> ans = new LinkedList<>();
if (root == null) {
return ans;
}
Queue<TreeNode> queue = new LinkedList<>();
queue.offer(root);
while (!queue.isEmpty()) {
List<Integer> temp = new LinkedList<>();
int size = queue.size();
for (int i = 0; i < size; i++) {
TreeNode node = queue.poll();
temp.add(node.val);
if (node.left != null) {
queue.offer(node.left);
}
if (node.right != null) {
queue.offer(node.right);
}
}
ans.add(0, new LinkedList<>(temp));
}
return ans;
}
public static void main(String[] args) {
TreeNode n1 = new TreeNode(3);
TreeNode n2 = new TreeNode(9);
n1.left = n2;
TreeNode n3 = new TreeNode(20);
n1.right = n3;
TreeNode n4 = new TreeNode(15);
n2.left = null;
n2.right = null;
n3.left = n4;
TreeNode n5 = new TreeNode(7);
n3.right = n5;
levelOrderBottom(n1);
}
public static class TreeNode {
int val;
TreeNode left;
TreeNode right;
TreeNode() {
}
TreeNode(int val) {
this.val = val;
}
TreeNode(int val, TreeNode left, TreeNode right) {
this.val = val;
this.left = left;
this.right = right;
}
}
}
|
打印：
思路：
思路上，其实和z字形很类似。只不过借助于java的api实现头插入。以后可以试试Go或者C的写法。