Binary Tree Zigzag Level Order Traversal

[解题思路]

二叉树的层序遍历通过使用queue来实现，

上网搜索了下，发现该题可以使用栈来解决，通过分析执行结果确实是后进先出

这里通过定义leftToRight来表示是从左到右还是从右到左

从左到右：先加left后加right

从右到左：先加right后加left

orderlevel traversal 遍历最后每行添加到result前 判断是leftToRight 还是Right to Left

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ArrayList<ArrayList<Integer>> zigzagLevelOrder(TreeNode root) {
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        
        if(root == null)
            return result;
            
        LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
        queue.add(root);
        boolean leftToRight = true;
        int nextLevelCount = 1;
        
        while(!queue.isEmpty()){
            int curLevel = nextLevelCount;
            nextLevelCount = 0;
            ArrayList<Integer> lvl = new ArrayList<Integer>();
            
            for(int i = 0; i < curLevel; i++){
                TreeNode tmp = queue.poll();
                lvl.add(tmp.val);
                if(tmp.left != null){
                    queue.add(tmp.left);
                    nextLevelCount++;
                }
                if(tmp.right != null){
                    queue.add(tmp.right);
                    nextLevelCount++;
                }
            }
            if(leftToRight){
                result.add(lvl);
                leftToRight = !leftToRight;
            }else{
                ArrayList<Integer> reverseLvl = new ArrayList<Integer>();
                for(int i = lvl.size()-1; i >= 0; i--){
                    reverseLvl.add(lvl.get(i));
                }
                result.add(reverseLvl);
                leftToRight = !leftToRight;
            }
            
        }
        return result;
    }
}

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ArrayList<ArrayList<Integer>> zigzagLevelOrder(TreeNode root) {
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        if(root == null){
            return result;            
        }
        Stack<TreeNode> curLvl = new Stack<TreeNode>();
        Stack<TreeNode> nextLvl = new Stack<TreeNode>();
        boolean leftToRight = true;
        curLvl.push(root);
        ArrayList<Integer> output = new ArrayList<Integer>();
        while(!curLvl.empty()){
            TreeNode curNode = curLvl.pop();
            output.add(curNode.val);
            if(leftToRight){
                if(curNode.left != null){
                    nextLvl.add(curNode.left);
                }
                if(curNode.right != null){
                    nextLvl.add(curNode.right);
                }
            } else{
                if(curNode.right != null){
                    nextLvl.add(curNode.right);
                }
                if(curNode.left != null){
                    nextLvl.add(curNode.left);
                }
            }
            if(curLvl.empty()){
                result.add(output);
                output = new ArrayList<Integer>();
                leftToRight = !leftToRight;
                curLvl.addAll(nextLvl);
                nextLvl.clear();
            }
        }
        return result;
    }
}