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题目及测试
package pid102;
import java.util.List;
/*二叉树的层次遍历
给定一个二叉树,返回其按层次遍历的节点值。 (即逐层地,从左到右访问所有节点)。
例如:
给定二叉树: [3,9,20,null,null,15,7],
3
/ \
9 20
/ \
15 7
返回其层次遍历结果:
[
[3],
[9,20],
[15,7]
]
}*/
public class main {
public static void main(String[] args) {
Object[] x=new Object[]{1,2,2,3,4,4,3};
BinaryTree tree=new BinaryTree(x);
tree.printTree(tree.root);
test(tree.root);
Object[] x2=new Object[]{3,9,20,null,null,15,7};
BinaryTree tree2=new BinaryTree(x2);
tree2.printTree(tree2.root);
test(tree2.root);
}
private static void test(TreeNode ito) {
Solution solution = new Solution();
List<List<Integer>> rtn;
long begin = System.currentTimeMillis();
rtn = solution.levelOrder(ito);//执行程序
long end = System.currentTimeMillis();
System.out.println("rtn=" );
for (List<Integer> list : rtn) {
for (Integer integer : list) {
System.out.print(integer+" ");
}
System.out.println();
}
System.out.println();
System.out.println("耗时:" + (end - begin) + "ms");
System.out.println("-------------------");
}
}
解法1(成功,2ms,很快)
建立一个queue,一个rowlist,
一个现在层次的节点数num,一个下一层next
将root加入queue
当queue不为空
从queue取出头,num–,将值加入rowlist,将left,right加入queue尾部,next++
当num为0 则num=next,next=0,结果加入rowlist,rowlist新建一个
package pid102;
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public List<List<Integer>> levelOrder(TreeNode root) {
List<List<Integer>> result=new ArrayList<List<Integer>>();
List<Integer> rowResult=new ArrayList<Integer>();
if(root==null){
return result;
}
int num=1;
int next=0;
Queue<TreeNode> queue=new LinkedList<TreeNode>();
queue.add(root);
while (!queue.isEmpty()) {
TreeNode now=queue.poll();
rowResult.add(now.val);
num--;
if(now.left!=null){
queue.add(now.left);
next++;
}
if(now.right!=null){
queue.add(now.right);
next++;
}
if(num==0){
num=next;
next=0;
result.add(rowResult);
rowResult=new ArrayList<Integer>();
}
}
return result;
}
}