a simple java solution

public class Solution {
TreeNode pre=null;
public TreeNode Convert(TreeNode pRootOfTree) {
if(pRootOfTree==null)
return null;
Convert(pRootOfTree.right);
if(pre==null)
pre=pRootOfTree;
else{
pRootOfTree.right=pre;
pre.left=pRootOfTree;
pre=pRootOfTree;
}
Convert(pRootOfTree.left);
return pre;
}
}