LeetCode(103)——Binary Tree Zigzag Level Order Traversal

題目：

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,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its zigzag level order traversal as:

[
  [3],
  [20,9],
  [15,7]
]

和102類似，只不過偶數層的順序是反過來的。

 

AC:

public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        
        List<List<Integer>> resultList = new LinkedList<>();
        
        if (root == null) {
            return resultList;
        }
        
        List<TreeNode> tmpList = new LinkedList<>();
        tmpList.add(root);
        
        boolean order = true;
        
        while (!tmpList.isEmpty()) {
            List<Integer> valueList = new LinkedList<>();
            int size = tmpList.size();

            for (int i = 0; i < size; i++) {
                TreeNode node = tmpList.get(0);
                if (node.left != null) {
                    tmpList.add(node.left);
                }
                if (node.right != null) {
                    tmpList.add(node.right);
                }

                if (order) {
                    valueList.add(node.val);
                } else {
                    valueList.add(0, node.val);
                }
                
                tmpList.remove(0);
            }

            
            order = !order;
            resultList.add(valueList);
          
        }
        
        return resultList;
    }

迭代，3ms

 

public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> resultList = new ArrayList<>();
        helper(resultList, root, 1);
        return resultList;

    }

    private void helper(List<List<Integer>> resultList, TreeNode root, int depth) {
        if (root == null) {
            return;
        }
        
        if (resultList.size() < depth) {
            resultList.add(new LinkedList<>());
        }
        
        if ((depth & 1) != 0) {
            resultList.get(depth - 1).add(root.val);
        } else {
            resultList.get(depth - 1).add(0, root.val);
        }
        
        helper(resultList, root.left, depth + 1);
        helper(resultList, root.right, depth + 1);
    }

遞歸2ms。當奇數層時，按順序插入，偶數層時，右邊所有的數插入在左邊所有數的前面。