Java实现二叉树自底向上的层序遍历

Java实现二叉树自底向上的层序遍历

例如，给定二叉树 [3,9,20,null,null,15,7],

返回结果

二叉树结构

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode() {}
    TreeNode(int val) { this.val = val; }
    TreeNode(int val, TreeNode left, TreeNode right) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}

实现方法（层次遍历）队列

while(size>0)循环意思为将该层遍历完，出队列，装入list中

	private static List<List<Integer>> levelOrderBottom(TreeNode root) {
        LinkedList<List<Integer>> res = new LinkedList<>();
        if (root == null)
            return res;
        TreeNode p = root;
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.offer(p);
        while (!queue.isEmpty()) {
            int size = queue.size();
            List<Integer> temp = new LinkedList<Integer>();
            while (size > 0) {
                p = queue.poll();
                temp.add(p.val);
                if (p.left != null) queue.offer(p.left);
                if (p.right != null) queue.offer(p.right);
                size--;
            }
            res.addFirst(temp);
        }
        return res;
    }