LeetCode107-二叉树的层序遍历II

LeetCode107-二叉树的层序遍历II

直接使用链表头插法返回

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public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> res = new LinkedList<>();
        if (root == null) return res;
        Deque<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            int n = queue.size();
            List<Integer> level = new LinkedList<>();
            for (int i = 0; i < n; i++) {
                TreeNode poll = queue.poll();
                level.add(poll.val);
                TreeNode left = poll.left, right = poll.right;
                if (left != null) {
                    queue.offer(poll.left);
                }
                if (right != null) {
                    queue.offer(poll.right);
                }
            }
            res.add(0, level);
        }
        return res;
    }

借助栈

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public List<List<Integer>> levelOrderBottom(TreeNode root) {
    List<List<Integer>> res = new LinkedList<>();
      if (root == null) return res;
      Queue<TreeNode> queue = new LinkedList<>();
      Stack<List<Integer>> stack = new Stack<>();
      queue.offer(root);
      while (!queue.isEmpty()) {
          int n = queue.size();
          List<Integer> level = new LinkedList<>();
          for (int i = 0; i < n; i++) {
              TreeNode poll = queue.poll();
              level.add(poll.val);
              if (poll.left != null) {
                  queue.offer(poll.left);
              }
              if (poll.right != null) {
                  queue.offer(poll.right);
              }
          }
          stack.push(level);
      }
      while (!stack.isEmpty()){
          res.add(stack.pop());
      }
      return res;
}