二叉树的层序遍历
解法一:利用队列遍历二叉树
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;
public class LeetCode_102 {
/**
* 利用队列遍历二叉树:队列的特性是先进先出
* @param root
* @return
*/
public static List<List<Integer>> levelOrder(TreeNode root) {
if (root == null) {
return new ArrayList<>();
}
List<List<Integer>> result = new ArrayList<>();
// 暂存每一行的节点
Queue<TreeNode> nodes = new LinkedList<>();
nodes.add(root);
while (!nodes.isEmpty()) {
// 每次遍历一行数据
List<Integer> vals = new ArrayList<>();
int count = nodes.size();
while (count > 0) {
TreeNode curNode = nodes.poll();
vals.add(curNode.val);
// 将当前节点的左右子节点从左到右放入队列
if (curNode.left != null) {
nodes.add(curNode.left);
}
if (curNode.right != null) {
nodes.add(curNode.right);
}
count--;
}
result.add(vals);
}
return result;
}
public static void main(String[] args) {
TreeNode root = new TreeNode(3);
root.left = new TreeNode(9);
root.right = new TreeNode(20);
root.right.left = new TreeNode(15);
root.right.right = new TreeNode(7);
for (List<Integer> integers : levelOrder(root)) {
for (Integer integer : integers) {
System.out.print(integer + " ");
}
System.out.println();
}
}
}