【一次过】Lintcode 71. 二叉树的锯齿形层次遍历

给出一棵二叉树，返回其节点值的锯齿形层次遍历（先从左往右，下一层再从右往左，层与层之间交替进行） 

样例

给出一棵二叉树 {3,9,20,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

返回其锯齿形的层次遍历为：

[
  [3],
  [20,9],
  [15,7]
]

---

解题思路：

与Lintcode 69:Binary Tree Level Order Traversal思路一致，只是加了一个标志位flag用来隔层翻转。

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */

public class Solution {
    /**
     * @param root: A Tree
     * @return: A list of lists of integer include the zigzag level order traversal of its nodes' values.
     */
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        // write your code here
        List<List<Integer>> res = new ArrayList<>();
        if(root == null)
            return res;
            
        boolean flag = false;
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        
        while(!queue.isEmpty()){
            int len = queue.size();
            List<Integer> list = new ArrayList<>();
            
            for(int i=0; i<len; i++){
                TreeNode temp = queue.poll();
                
                list.add(temp.val);

                if(temp.left != null)
                    queue.offer(temp.left);
                if(temp.right != null)
                    queue.offer(temp.right);
            }
            
            if(flag)
                Collections.reverse(list);
            
            flag = !flag;
            res.add(list);
        }
        
        
        return res;
    }
}

用boolean变量实现，一层在尾部添加元素，一层在头部添加元素，达到锯齿形的效果。

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */

public class Solution {
    /**
     * @param root: A Tree
     * @return: A list of lists of integer include the zigzag level order traversal of its nodes' values.
     */
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        // write your code here
        List<List<Integer>> res = new ArrayList<>();
        if(root == null)
            return res;
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        boolean isReverse = false;//是否翻转
        
        while(!queue.isEmpty()){
            int len = queue.size();
            List<Integer> list = new ArrayList<>();
            
            for(int i=0; i<len; i++){
                TreeNode node = queue.poll();
                if(!isReverse)
                    list.add(node.val);
                else
                    list.add(0, node.val);
                
                if(node.left != null)
                    queue.offer(node.left);
                if(node.right != null)
                    queue.offer(node.right);
            }
            
            res.add(list);
            isReverse = !isReverse;
        }
        
        return res;
    }
}