用一个队列来保存将要打印的结点。为了把二叉树的每一行单独打印到一行里,我们需要两个变量:一个变量表示在当前的层中还没有打印的结点数,另一个变量表示下一次结点的数目。
private static class BinaryTreeNode { private int val; private BinaryTreeNode left; private BinaryTreeNode right; public BinaryTreeNode() { } public BinaryTreeNode(int val) { this.val = val; } @Override public String toString() { return val + ""; } }
import java.util.LinkedList; import java.util.List; public class Test60 { private static class BinaryTreeNode { private int val; private BinaryTreeNode left; private BinaryTreeNode right; public BinaryTreeNode() { } public BinaryTreeNode(int val) { this.val = val; } @Override public String toString() { return val + ""; } } /** * 题目:从上到下按层打印二叉树,同一层的结点按从左到右的顺序打印,每一层打印一行。 * @param root */ public static void print(BinaryTreeNode root) { if (root == null) { return; } List<BinaryTreeNode> list = new LinkedList<>(); BinaryTreeNode node; // 当前层的结点个数 int current = 1; // 记录下一层的结点个数 int next = 0; list.add(root); while (list.size() > 0) { node = list.remove(0); current--; System.out.printf("%-3d", node.val); if (node.left != null) { list.add(node.left); next++; } if (node.right != null) { list.add(node.right); next++; } if (current ==0) { System.out.println(); current = next; next = 0; } } } public static void main(String[] args) { BinaryTreeNode n1 = new BinaryTreeNode(1); BinaryTreeNode n2 = new BinaryTreeNode(2); BinaryTreeNode n3 = new BinaryTreeNode(3); BinaryTreeNode n4 = new BinaryTreeNode(4); BinaryTreeNode n5 = new BinaryTreeNode(5); BinaryTreeNode n6 = new BinaryTreeNode(6); BinaryTreeNode n7 = new BinaryTreeNode(7); BinaryTreeNode n8 = new BinaryTreeNode(8); BinaryTreeNode n9 = new BinaryTreeNode(9); n1.left = n2; n1.right = n3; n2.left = n4; n2.right = n5; n3.left = n6; n3.right = n7; n4.left = n8; n4.right = n9; print(n1); } }