已知先序中序求后序Java

可以使用递归的方式来实现先序和中序求后序，具体实现代码如下：

public class Tree {
    private static class Node {
        private char value;
        private Node left;
        private Node right;

        public Node(char value) {
            this.value = value;
        }
    }

    public static String preInToPost(String preOrder, String inOrder) {
        if (preOrder == null || inOrder == null || preOrder.length() != inOrder.length()) {
            return null;
        }
        Node root = preInToPost(preOrder.toCharArray(), 0, preOrder.length() - 1, inOrder.toCharArray(), 0, inOrder.length() - 1);
        StringBuilder sb = new StringBuilder();
        postOrder(root, sb);
        return sb.toString();
    }

    private static Node preInToPost(char[] preOrder, int preStart, int preEnd, char[] inOrder, int inStart, int inEnd) {
        if (preStart > preEnd) {
            return null;
        }
        Node root = new Node(preOrder[preStart]);
        int index = findIndex(inOrder, inStart, inEnd, preOrder[preStart]);
        int leftSize = index - inStart;
        root.left = preInToPost(preOrder, preStart + 1, preStart + leftSize, inOrder, inStart, index - 1);
        root.right = preInToPost(preOrder, preStart + leftSize + 1, preEnd, inOrder, index + 1, inEnd);
        return root;
    }

    private static void postOrder(Node root, StringBuilder sb) {
        if (root == null) {
            return;
        }
        postOrder(root.left, sb);
        postOrder(root.right, sb);
        sb.append(root.value);
    }

    private static int findIndex(char[] arr, int start, int end, char target) {
        for (int i = start; i <= end; i++) {
            if (arr[i] == target) {
                return i;
            }
        }
        return -1;
    }

    public static void main(String[] args) {
        String preOrder = "ABDCEGF";
        String inOrder = "DBAEGCF";
        String postOrder = preInToPost(preOrder, inOrder);
        System.out.println(postOrder); // DCBGEFA
    }
}

该代码中使用了一个内部类 `Node` 表示二叉树的节点，`preInToPost` 方法用于递归构建二叉树，`postOrder` 方法用于递归遍历二叉树并输出后序遍历序列，`findIndex` 方法用于查找中序遍历序列中某个元素的位置。最后在 `main` 方法中调用 `preInToPost` 方法并输出结果。