24.二叉树-给前中求后序

分四步走 
（1）确定树的根节点。树根是当前树中所有元素在先序遍历中最先出现的元素，即先序遍历的第一个结点就是二叉树的跟 
（2）求解树的子树。找到根在中序遍历的位置，位置左边就是二叉树的左孩子，位置右边是二叉树的右孩子，如果根结点左边或右边为空，那么该方向子树为空；如果根节点左边和右边都为空，那么根节点已经为叶子节点。 
（3）对二叉树的左、右孩子分别进行步骤（1）（2），直到求出二叉树的结构为止。

（4）后序遍历二叉树。

 

package facehandjava.tree;

import java.util.Stack;

public class RebuildTree {
    public static void main(String[] args) {
        int[] before = {6, 3, 1, 2, 5, 4, 9, 7, 8};
        int[] middle = {1, 2, 3, 4, 5, 6, 7, 8, 9};
        Node rebuildTree = RebuildTree(before, middle);
        afterTree(rebuildTree);
    }

    public static Node RebuildTree(int[] before, int[] middle) {
        int l = before.length;
        if (l != middle.length || l == 0) {
            return null;
        }
        int i = 0;
        while (before[0] != middle[i]) {
            i++;
        }
        Node node = new Node(before[0], null, null);
        int[] b_left = new int[i];
        int[] m_left = new int[i];
        int[] b_right = new int[l - i - 1];
        int[] m_right = new int[l - i - 1];
        for (int j = 0; j < l; j++) {
            if (j < i) {
                b_left[j] = before[j + 1];
                m_left[j] = middle[j];
            } else if (j > i) {
                b_right[j - i - 1] = before[j];
                m_right[j - i - 1] = middle[j];
            }
        }
        node.setLeftNode(RebuildTree(b_left, m_left));
        node.setRightNode(RebuildTree(b_right, m_right));
        return node;
    }

//    public static void afterTree(Node node) {
//        if (node == null) {
//            return;
//        }
//        afterTree(node.getLeftNode());
//        afterTree(node.getRightNode());
//        System.out.print(node.getData());
//    }

    public static void afterTree(Node node) {
        Stack<Node> stack = new Stack<>();
        Stack<Node> out = new Stack<>();
        while (node != null || !stack.isEmpty()) {
            if (node != null) {
                stack.push(node);
                out.push(node);
                node = node.getRightNode();
            } else {
                node = stack.pop();
                node = node.getLeftNode();
            }
        }
        while (out.size() > 0) {
            System.out.print(out.pop().getData());
        }
    }

}