public class Solution {
    private TreeNode head = null, pre = null;
    public TreeNode Convert(TreeNode pRootOfTree) {
        inOrder(pRootOfTree);
        return head;
    }
    public void inOrder(TreeNode node) {
        if (node==null) return;
        inOrder(node.left);
        if (head==null) head = node;
        node.left = pre;
        if (pre!=null) pre.right = node;
        pre = node;
        inOrder(node.right);
        
    }
}