題目：從上往下打印出二叉樹的每個(gè)結(jié)點(diǎn)双吆，同一層的結(jié)點(diǎn)按照從左向右的順序打印。

代碼如下：

package demo;

import java.util.LinkedList;
import java.util.Queue;

public class Test23 {
    public static class BinaryTreeNode {
        int value;
        BinaryTreeNode left;
        BinaryTreeNode right;
    }

    public static void printFromTopToBottom(BinaryTreeNode root) {
        if (root != null) {
            // 存放還未遍歷的元素
            Queue<BinaryTreeNode> list = new LinkedList<>();
            // 將根節(jié)點(diǎn)入隊(duì)
            list.add(root);
            // 記錄當(dāng)前處理的結(jié)點(diǎn)
            BinaryTreeNode curNode;
            // 如果隊(duì)列非空绷落，則一直進(jìn)行處理
            while (!list.isEmpty()) {
                // 刪除對(duì)首元素
                curNode = list.remove();
                // 輸出對(duì)首元素的值
                System.out.print(curNode.value + " ");
                // 如果左子結(jié)點(diǎn)不為空，則左子結(jié)點(diǎn)入隊(duì)
                if (curNode.left != null) {
                    list.add(curNode.left);
                }
                // 如果右子結(jié)點(diǎn)不為空始苇，則右子結(jié)點(diǎn)入隊(duì)
                if (curNode.right != null) {
                    list.add(curNode.right);
                }
            }
        }
    }
}

測(cè)試1：

測(cè)試1

public static void main(String[] args) {
    BinaryTreeNode root = new BinaryTreeNode();
    root.value = 8;
    root.left = new BinaryTreeNode();
    root.left.value = 6;
    root.right = new BinaryTreeNode();
    root.right.value = 10;
    root.left.left = new BinaryTreeNode();
    root.left.left.value = 5;
    root.left.right = new BinaryTreeNode();
    root.left.right.value = 7;
    root.right.left = new BinaryTreeNode();
    root.right.left.value = 9;
    root.right.right = new BinaryTreeNode();
    root.right.right.value = 11;
    printFromTopToBottom(root);
}

運(yùn)行結(jié)果

測(cè)試2：

測(cè)試2

public static void main(String[] args) {
    BinaryTreeNode root2 = new BinaryTreeNode();
    root2.value = 1;
    root2.left = new BinaryTreeNode();
    root2.left.value = 3;
    root2.left.left = new BinaryTreeNode();
    root2.left.left.value = 5;
    root2.left.left.left = new BinaryTreeNode();
    root2.left.left.left.value = 7;
    root2.left.left.left.left = new BinaryTreeNode();
    root2.left.left.left.left.value = 9;
    printFromTopToBottom(root2);
}

運(yùn)行結(jié)果

測(cè)試3：

測(cè)試3

public static void main(String[] args) {
    BinaryTreeNode root3 = new BinaryTreeNode();
    root3.value = 0;
    root3.right = new BinaryTreeNode();
    root3.right.value = 2;
    root3.right.right = new BinaryTreeNode();
    root3.right.right.value = 4;
    root3.right.right.right = new BinaryTreeNode();
    root3.right.right.right.value = 6;
    root3.right.right.right.right = new BinaryTreeNode();
    root3.right.right.right.right.value = 8;
    printFromTopToBottom(root3);
}

運(yùn)行結(jié)果

測(cè)試4：

public static void main(String[] args) {
    BinaryTreeNode root3 = new BinaryTreeNode();
    root3.value = 1;
    printFromTopToBottom(root3);
}

運(yùn)行結(jié)果

測(cè)試5：

public static void main(String[] args) {
    printFromTopToBottom(null);
}

來源：http://blog.csdn.net/derrantcm/article/details/46705719