二叉树的前序,中序,后序的遍历(非递归/递归实现)

 1.非递归遍历二叉树 

  非递归实现需要手动控制压栈的过程,目的既要保证每个节点都访问到且能有效控制程序结束 

    //先序遍历 先push right再push left,目的是先处理left节点 弹栈就打印
    public static void prefix(Node node) {
        System.out.print("pre-order: ");
        Stack stack = new Stack<>();
        stack.push(node);
        while (!stack.empty()) {
            node = stack.pop();
            System.out.print(node.val + " ");
            if (node.right != null) {
                stack.push(node.right);
            }
            if (node.left != null) {
                stack.push(node.left);
            }
        }
        System.out.println();
    }

    //中序遍历: 只要left不为null一直往左窜直到null,并将沿途经过的节点压入栈,
    //然后弹出即打印,再往右窜,不为空就往左窜,压栈...
    public static void infill(Node node) {
        System.out.print("infill-order: ");
        Stack stack = new Stack<>();
        //先不急压入头节点
        while (!stack.isEmpty() || node != null){
            if(node != null){
                stack.push(node);
                node = node.left;
            }else{
                node = stack.pop();
                System.out.print(node.val + " ");
                node = node.right;
            }
        }
        System.out.println();
    }

    //后续遍历打印顺序:左 右 头 -> 逆序看: 头 右 左,那么就需要先push左才能先pop右
    public static void post(Node node) {
        System.out.print("post-order: ");
        Stack stack = new Stack<>();
        Stack data = new Stack<>();
        stack.push(node);
        while (!stack.empty()) {
            node = stack.pop();
            data.push(node);
            if (node.left != null) {
                stack.push(node.left);
            }
            if (node.right != null) {
                stack.push(node.right);
            }
        }
        while (!data.empty()){
            System.out.print(data.pop().val + " ");
        }
        System.out.println();
    }

 2.递归遍历二叉树 

    public static void pref(Node node){
        if(node == null){
            return;
        }
        System.out.print(node.val + "t");
        pref(node.left);
        pref(node.right);
    }

    public static void infill(Node node){
        if(node == null){
            return;
        }
        infill(node.left);
        System.out.print(node.val + "t");
        infill(node.right);
    }

    public static void post(Node node){
        if(node == null){
            return;
        }
        post(node.left);
        post(node.right);
        System.out.print(node.val + "t");
    }

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