根据二叉树的前序和中序获得后序，后序和中序获得前序的实现。可以用递归，呵呵！

下面是我写的测试程序：

package algorithm;

public class BTree {

 private static StringBuilder postOrderReverse = new StringBuilder();// 储存后序的一个逆行
 private static StringBuilder preOrder = new StringBuilder();// 储存前序

 /**
  * 根据先序和中序，获得后序
  *
  * @param preOrder
  * @param inOrder
  */
 public static void getPostOrder(String preOrder, String inOrder) {

  if (preOrder.length() == 1 && inOrder.length() == 1) {
   postOrderReverse.append(preOrder);
   return;
  }

  char root = preOrder.charAt(0);
  int rootIndex = -1;
  String left_InOrder = null;
  String left_preOrder = null;
  String right_Inorder = null;
  String right_preOrder = null;

  postOrderReverse.append(root);// 将跟节点放入

  rootIndex = inOrder.indexOf(root);
  left_InOrder = inOrder.substring(0, rootIndex);// 左子树 的中序
  right_Inorder = inOrder.substring(rootIndex + 1);// 右子树的中序

  preOrder = preOrder.substring(1);
  int size = left_InOrder.length();
  int index = -1;
  for (int i = 0; i < size; ++i) {
   int tmp = preOrder.indexOf(left_InOrder.charAt(i));
   if (index < tmp)
    index = tmp;
  }

  left_preOrder = preOrder.substring(0, index + 1);// 左子树前序
  right_preOrder = preOrder.substring(index + 1);// 右子树前序

  if (right_preOrder != null && !right_Inorder.equals(""))
   getPostOrder(right_preOrder, right_Inorder);

  if (left_preOrder != null && !left_preOrder.equals(""))
   getPostOrder(left_preOrder, left_InOrder);
 }

 /**
  * 根据后序和中序 ，获得先序
  *
  * @param postOrder
  * @param inOrder
  */
 public static void getPreOrder(String postOrder, String inOrder) {

  if (postOrder.length() == 1 && inOrder.length() == 1) {
   preOrder.append(postOrder);
   return;
  }

  String root = postOrder.substring(postOrder.length() - 1);
  int rootIndex = -1;
  String left_InOrder = null;
  String left_postOrder = null;
  String right_Inorder = null;
  String right_postOrder = null;

  preOrder.append(root);

  rootIndex = inOrder.indexOf(root);
  left_InOrder = inOrder.substring(0, rootIndex);// 左子树中序
  right_Inorder = inOrder.substring(rootIndex + 1);// 右子树中序

  int index = -1;
  int size = left_InOrder.length();
  for (int i = 0; i < size; ++i) {
   int tmp = postOrder.indexOf(left_InOrder.charAt(i));
   if (tmp > index)
    index = tmp;
  }

  left_postOrder = postOrder.substring(0, index + 1);// 左 子树后序
  right_postOrder = postOrder
    .substring(index + 1, postOrder.length() - 1);// 右子树后序

  if (left_InOrder != null && !left_InOrder.equals("")
    && left_postOrder != null && !left_postOrder.equals(""))
   getPreOrder(left_postOrder, left_InOrder);
  if (right_Inorder != null && !right_Inorder.equals("")
    && right_postOrder != null && !right_postOrder.equals(""))
   getPreOrder(right_postOrder, right_Inorder);
 }

 public static void main(String[] args) {
  getPostOrder("BCAD", "CBAD");
  System.out.println(postOrderReverse.reverse());
  getPreOrder("ACBFGED", "ABCDEFG");
  System.out.println(preOrder);
 }
}