【剑指 Offer 32 - I II III】从上到下打印二叉树 I II III

题目

剑指 Offer 32 - I. 从上到下打印二叉树

剑指 Offer 32 - II. 从上到下打印二叉树 II

剑指 Offer 32 - III. 从上到下打印二叉树 III

解题思路

三道题思路相同，只不过处理格式不同

准备一个队列，将根节点入栈，将根节点弹出的同时记录根节点的值并且将根节点的左右子树入队，以此类推，直到队列中无任何元素

代码

剑指 Offer 32 - I. 从上到下打印二叉树

class Solution {

    private Queue<TreeNode> queue = new LinkedList<>();

    public int[] levelOrder(TreeNode root) {
        if(root==null){
            return new int[]{};
        }
        queue.add(root);
        List<Integer> list = new ArrayList<>();
        while (!queue.isEmpty()) {
            int count = queue.size();
            while (count > 0) {
                list.add(queue.peek().val);
                TreeNode node = queue.poll();
                if (node.left != null) {
                    queue.add(node.left);
                }
                if (node.right != null) {
                    queue.add(node.right);
                }
                count--;
            }
        }
        return list.stream().mapToInt(Integer::valueOf).toArray();
    }

}

剑指 Offer 32 - II. 从上到下打印二叉树 II

/**
 * 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>> answerLists = new ArrayList<>();


    public List<List<Integer>> levelOrder(TreeNode root) {
        if (root == null) {
            return answerLists;
        }
        solve(root);
        return answerLists;
    }

    public void solve(TreeNode root) {
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while (!queue.isEmpty()) {
            int count = queue.size();
            List<Integer> list = new ArrayList<Integer>();
            while (count > 0) {
                TreeNode node = queue.poll();
                list.add(node.val);
                if (node.left != null) {
                    queue.add(node.left);
                }
                if (node.right != null) {
                    queue.add(node.right);
                }
                count--;
            }
            answerLists.add(list);
        }
    }
}

---

剑指 Offer 32 - III. 从上到下打印二叉树 III

class Solution {

    public List<List<Integer>> answerLists = new ArrayList<>();


    public List<List<Integer>> levelOrder(TreeNode root) {
        if (root == null) {
            return answerLists;
        }
        List<List<Integer>> answer=new ArrayList<>();
        solve(root);
        int i = 0;
        for (List<Integer> ans : answerLists) {
            if (i % 2 != 0) {
               Collections.reverse(ans);
                answer.add(ans);
            }else{
                answer.add(ans);
            }
             i++;
        }
        return answer;
    }

    public void solve(TreeNode root) {
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while (!queue.isEmpty()) {
            int count = queue.size();
            List<Integer> list = new ArrayList<Integer>();
            while (count > 0) {
                TreeNode node = queue.poll();
                list.add(node.val);
                if (node.left != null) {
                    queue.add(node.left);
                }
                if (node.right != null) {
                    queue.add(node.right);
                }
                count--;
            }
            answerLists.add(list);
        }
    }

}