深入解析：三种二叉树非递归遍历方法及其代码实现

public static void preOrderUnRecur(Node head) {
    if (head == null) return;
    Stack<Node> stack = new Stack<>();
    stack.push(head);
    while (!stack.isEmpty()) {
        Node cur = stack.pop();          // 弹出并处理
        System.out.print(cur.value + " ");
        if (cur.right != null)  stack.push(cur.right);
        if (cur.left  != null)  stack.push(cur.left);
    }
}

public static void inOrderUnRecur(Node head) {
    Stack<Node> stack = new Stack<>();
    Node cur = head;
    while (!stack.isEmpty() || cur != null) {
        if (cur != null) {
            stack.push(cur);
            cur = cur.left;
        } else {
            cur = stack.pop();
            System.out.print(cur.value + " ");
            cur = cur.right;
        }
    }
}

public static void posOrderUnRecur(Node head) {
    if (head == null) return;
    Stack<Node> s1 = new Stack<>(), s2 = new Stack<>();
    s1.push(head);
    while (!s1.isEmpty()) {
        Node cur = s1.pop();
        s2.push(cur);
        if (cur.left  != null) s1.push(cur.left);
        if (cur.right != null) s1.push(cur.right);
    }
    while (!s2.isEmpty()) {
        System.out.print(s2.pop().value + " ");
    }
}