完整程式碼地址

題目

從上到下按層列印二叉樹，同一層結點從左至右輸出。每一層輸出一行。

思路

用佇列來儲存要列印的節點。
同時我們需要兩個變數：一個變量表示在當前層中還沒有列印的節點數；另一個變量表示下一層節點的數目。

程式碼

public class _32_02_PrintTreesInLines {

    public static class TreeNode {
        public int val = 0;
        public TreeNode left = null;
        public TreeNode right = null
 
;
        public TreeNode(int val) {
            this.val = val;
        }
    }

    public static ArrayList<ArrayList<Integer>> Print(TreeNode pRoot) {
        if(pRoot == null)
            return new ArrayList<>();

        ArrayList<ArrayList<Integer>> result = new ArrayList<>();
        Queue<TreeNode> queue = new
 
 LinkedList<>();
        queue.add(pRoot);

        ArrayList<Integer> lineResult = new ArrayList<>();
        int numOfThisLine = 1;
        int numOfNextLine = 0;

        while(!queue.isEmpty()) {
            TreeNode node = queue.poll();
            lineResult.add(node.val);
            --numOfThisLine;
            if
 
(node.left != null) {
                queue.add(node.left);
                ++numOfNextLine;
            }
            if(node.right != null) {
                queue.add(node.right);
                ++numOfNextLine;
            }
            if(numOfThisLine == 0) {
                result.add(lineResult);
                lineResult = new ArrayList<>();
                numOfThisLine = numOfNextLine;
                numOfNextLine = 0;
            }
        }
        return result;
    }
}

測試

public class _32_02_Test {

    public static void main(String[] args) {
        test1();
        test2();
        test3();
    }

    /**
     * 功能測試
     *     1
     *    / \
     *   2   3
     *  / \   \
     * 4   5   6
     */
    private static void test1() {
        TreeNode root = new TreeNode(1);
        TreeNode node2 = new TreeNode(2);
        TreeNode node3 = new TreeNode(3);
        TreeNode node4 = new TreeNode(4);
        TreeNode node5 = new TreeNode(5);
        TreeNode node6 = new TreeNode(6);
        root.left = node2;
        root.right = node3;
        node2.left = node4;
        node2.right = node5;
        node3.right = node6;
        System.out.println(_32_02_PrintTreesInLines.Print(root));
    }

    /**
     * 邊界測試
     * 1.只有一個節點
     * 2.每個節點都只有左子節點
     * 3.每個節點都只有右子節點
     */
    private static void test2() {
        TreeNode root = new TreeNode(1);

        System.out.println(_32_02_PrintTreesInLines.Print(root));

        TreeNode node2 = new TreeNode(2);
        TreeNode node3 = new TreeNode(3);
        root.left = node2;
        node2.left = node3;

        System.out.println(_32_02_PrintTreesInLines.Print(root));

        root.left = null;
        node2.left = null;
        root.right = node2;
        node2.right = node3;
        System.out.println(_32_02_PrintTreesInLines.Print(root));
    }

    /**
     * 極端測試 
     * null
     */
    private static void test3() {
        System.out.println(_32_02_PrintTreesInLines.Print(null));
    }

}