LeetCode-103-二叉树的锯齿形层序遍历

103. 二叉树的锯齿形层序遍历

难度中等450收藏分享切换为英文接收动态反馈

给定一个二叉树，返回其节点值的锯齿形层序遍历。（即先从左往右，再从右往左进行下一层遍历，以此类推，层与层之间交替进行）。

例如：
给定二叉树 [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

返回锯齿形层序遍历如下：

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

class Solution {
    public static List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> ans = new ArrayList<>();
        if (root == null) {
            return ans;
        }
        LinkedList<TreeNode> deque = new LinkedList<>();
        deque.add(root);
        int size = 0;
        boolean isHead = true;
        while (!deque.isEmpty()) {
            size = deque.size();
            List<Integer> curLevel = new ArrayList<>();
            for (int i = 0; i < size; i++) {
                TreeNode cur = isHead ? deque.pollFirst() : deque.pollLast();
                curLevel.add(cur.val);
                if(isHead) {
                    if (cur.left != null) {
                        deque.addLast(cur.left);
                    }
                    if (cur.right != null) {
                        deque.addLast(cur.right);
                    }
                }else {
                    if (cur.right != null) {
                        deque.addFirst(cur.right);
                    }
                    if (cur.left != null) {
                        deque.addFirst(cur.left);
                    }
                }
            }
            ans.add(curLevel);
            isHead = !isHead;
        }
        return ans;
    }
}

image-20210606142704542

递归写法

class Solution {

    List<List<Integer>> res = new ArrayList<>();

    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        if(root == null) return res;
        helper(root, 0, true);
        return res;
    }

    private void helper(TreeNode root, int level, boolean left){
        if(root == null) return;
        if(level >= res.size()) res.add(new ArrayList<Integer>());
        if(left) res.get(level).add(root.val);
        if(!left) res.get(level).add(0,root.val);
        helper(root.left, level+1, !left);
        helper(root.right, level+1, !left);
    }
}

image-20210606142858017