103. 二叉树的锯齿形层序遍历

103. 二叉树的锯齿形层序遍历

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        if (root == null)
            return new ArrayList();
        List<List<Integer>> ans = new ArrayList<>();
        Deque<TreeNode>[] stack = new Deque[2];
        stack[0] = new LinkedList<>();
        stack[1] = new LinkedList<>();
        int current = 0;
        int next = 1;
        stack[current].add(root);
        List<Integer> list = new ArrayList<>();
        while (!stack[0].isEmpty() || !stack[1].isEmpty()) {
            TreeNode node = stack[current].pop();
            list.add(node.val);
            if (current == 0) {
                if (node.left != null) {
                    stack[next].push(node.left);
                }
                if (node.right != null) {
                    stack[next].push(node.right);
                }
            } else {
                if (node.right != null) {
                    stack[next].push(node.right);
                } 
                if (node.left != null) {
                    stack[next].push(node.left);
                }
            }
            if (stack[current].isEmpty()) {
                ans.add(list);
                list = new ArrayList<Integer>();
                current = 1 - current;
                next = 1 - next;
            }
        }
        return ans;
    }
}