Java 代码

class Solution {
    public TreeNode convert(TreeNode root) {
        TreeNode[] pLast = new TreeNode[1];
        dfs(root, pLast);
        TreeNode head = pLast[0];
        while (head != null && head.left != null) {
            head = head.left;
        }
        return head;
    }
    private void dfs(TreeNode root, TreeNode[] pLast) {
        if (root == null) {
            return;
        }
        dfs(root.left, pLast);
        root.left = pLast[0];
        if (pLast[0] != null) {
            pLast[0].right = root;
        }
        pLast[0] = root;
        dfs(root.right, pLast);
    }
}