LeetCode 103 Binary Tree Zigzag Level Order Traversal

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).

For example:

Given binary tree

{3,9,20,#,#,15,7}

,

3
   / \
  9  20
    /  \
   15   7

return its zigzag level order traversal as:

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

思路：在层次遍历的基础上加上一个布尔型的flag

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
	public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
		boolean flag = true;
        List<Integer> list ;
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        LinkedList <TreeNode> linkedList = new LinkedList<TreeNode>();
        if(root == null) return result;
        TreeNode p;
        int num = 1;
        linkedList.addLast(root);
        while(!linkedList.isEmpty()) {
        	list = new ArrayList<Integer>();
            int i = 0;
            int temp = 0;
            while(i < num) {
                p = linkedList.pollFirst();
                if(flag) {
                	list.add(p.val);
                }else{
                	list.add(0,p.val);
                }
                if(p.left != null) {
                    linkedList.addLast(p.left);
                    temp++;
                }
                if(p.right != null) {
                    linkedList.addLast(p.right);
                    temp++;
                }
                
                i++;
            }
            flag=!flag;
            num = temp;
            result.add(list);
        }
        return result;
    }
}