binary-tree-preorder-traversal-2.java

// Binary Tree Preorder Traversal
// Morris先序遍历，时间复杂度O(n)，空间复杂度O(1)
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        ArrayList<Integer> result = new ArrayList<>();
        TreeNode cur = root;
        TreeNode prev = null;

        while (cur != null) {
            if (cur.left == null) {
                result.add(cur.val);
                prev = cur; /* cur刚刚被访问过 */
                cur = cur.right;
            } else {
                /* 查找前驱 */
                TreeNode node = cur.left;
                while (node.right != null && node.right != cur)
                    node = node.right;

                if (node.right == null) { /* 还没线索化，则建立线索 */
                    result.add(cur.val); /* 仅这一行的位置与中序不同 */
                    node.right = cur;
                    prev = cur; /* cur刚刚被访问过 */
                    cur = cur.left;
                } else {    /* 已经线索化，则删除线索  */
                    node.right = null;
                    /* prev = cur; 不能有这句，cur已经被访问 */
                    cur = cur.right;
                }
            }
        }
        return result;
    }
}