LintCode 71 -- 二叉树的锯齿形层次遍历

自己ac了一遍之后, 发现google排名靠前的少有Java版的实现, 这里贴上我自己的实现.

思路非常简单, 使用双端队列作为层次遍历的支持结构, 然后用一个布尔型变量控制下一层遍历是队头出还是队尾出.

public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: A list of lists of integer include 
     *          the zigzag level order traversal of its nodes' values 
     */
    public ArrayList> zigzagLevelOrder(TreeNode root) {
        // write your code here
        ArrayList> res = new ArrayList>();
        if (root == null) {
            return res;
        }

        ArrayList floor = new ArrayList<>();
        Deque deque = new ArrayDeque<>();
        TreeNode last = root;
        deque.offer(root);
        boolean nextPoll = true;

        while (!deque.isEmpty()) {
            TreeNode temp = nextPoll? deque.poll(): deque.pollLast();
            floor.add(temp.val);

            if (nextPoll) {
                if (temp.left != null) {
                    deque.offer(temp.left);
                }
                if (temp.right != null) {
                    deque.offer(temp.right);
                }
            } else {
                if (temp.right != null) {
                    deque.offerFirst(temp.right);
                }
                if (temp.left != null) {
                    deque.offerFirst(temp.left);
                }
            }

            if (last == temp) {
                res.add(floor);
                floor = new ArrayList<>();
                last = nextPoll? deque.peek(): deque.peekLast();
                nextPoll = !nextPoll;
            }
        }

        return res;
    }
}