让我们来写个算法吧，（2）二叉树的前序，中序，后序 遍历

public class TreeNode {
    
    TreeNode left;
    
    TreeNode right;
    
    int value;
    
    public TreeNode(int value ) {
        this.value = value;
    }

}

private TreeNode createTreeNode() {
        
        TreeNode[] node = new TreeNode[10];// 以数组形式生成一棵完全二叉树
        
        for (int i = 0; i < 10; i++) {
            node[i] = new TreeNode(i);
        }
        
        for (int i = 0; i < 10; i++) {
            if (i * 2 + 1 < 10)
                node[i].left = node[i * 2 + 1];
            if (i * 2 + 2 < 10)
                node[i].right = node[i * 2 + 2];
        }

        return node[0];
    }

private void preOrderRe(TreeNode node) {
        if(null == node) {
            return;
        }
        System.out.print(node.value+" ");
        preOrderRe(node.left);
        preOrderRe(node.right);
    }

private void midOrderRe(TreeNode node) {
        if(null == node) {
            return;
        }
        if(node.left!=null) {
            midOrderRe(node.left);
        }
        System.out.print(node.value+" ");
        
        midOrderRe(node.right);
    }

    private void postOrderRe(TreeNode node) {
        if(null == node) {
            return;
        }
        if(node.left!=null) {
            postOrderRe(node.left);
        }
        postOrderRe(node.right);
        
        System.out.print(node.value+" ");
        
    }