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问题分析

由于不论先序还是中序、后序遍历，在程序运行流程上没有本质区别，区别仅仅是访问语句的执行时间点。因此，只要有了先序的程序流程，改变一下访问语句代码位置，就可以实现前三种遍历方式。

先序遍历（非递归）

先序遍历是从树的根节点开始，依次访问左节点和右节点。非递归实现使用栈来模拟调用栈，确保每个节点只被访问一次。

public void visitFirst_N(Node tree) {    if (tree != null) {        Stack
   
     stack = new Stack<>();        stack.push(tree);        Node current = tree;        while (current != null) {            System.out.print(current.value + " ");            current = current.left;            if (current != null) {                stack.push(current);            } else {                while (!stack.isEmpty()) {                    Node node = stack.pop();                    current = node.right;                    if (current != null) {                        stack.push(current);                        break;                    }                }            }        }    }}
   

中序遍历（非递归）

中序遍历是从树的根节点开始，先访问左节点，后访问右节点。非递归实现仍然使用栈，但处理方式有所不同。

public void midTest(Node tree) {    if (tree != null) {        Stack
   
     stack = new Stack<>();        stack.push(tree);        Node current = tree;        while (current != null) {            current = current.left;            if (current != null) {                stack.push(current);            } else {                while (!stack.isEmpty()) {                    Node node = stack.pop();                    System.out.print(node.value + " ");                    current = node.right;                    if (current != null) {                        stack.push(current);                        break;                    }                }            }        }    }}
   

后序遍历（非递归）

后序遍历是从树的根节点开始，先访问右节点，再访问左节点。由于栈的先进后出特性，需要额外处理栈中的节点，确保左节点在栈底部。

public void LastTest(Node tree) {    if (tree != null) {        Stack
   
     stack = new Stack<>();        stack.push(tree);        Node current = tree;        while (current != null) {            current = current.left;            if (current != null) {                stack.push(current);            } else {                while (!stack.isEmpty()) {                    Node node = stack.peek();                    current = node.right;                    if (current == null) {                        Node x = stack.pop();                        System.out.print(x.value + " ");                        Node y = stack.pop();                        if (y.left == x) {                            stack.push(y);                            current = y.right;                        } else {                            Node k = stack.peek();                            System.out.print(y.value + " ");                            if (k.right == y) {                                while (!stack.isEmpty()) {                                    System.out.print(stack.pop().value + " ");                                }                                return;                            }                            current = k.right;                        }                    }                    if (current != null) {                        stack.push(current);                        break;                    }                }            }        }    }}
   

层次遍历（非递归）

层次遍历是按层访问树节点。使用队列来实现，先处理队列中的节点，输出其值，然后将左孩子和右孩子入队。

private void visitLevel(Node x) {    Queue
   
     queue = new Queue<>();    while (x != null) {        System.out.print(x.value + " ");        if (x.left != null) {            queue.enqueue(x.left);        }        if (x.right != null) {            queue.push(x.right);        }        x = queue.deQueue();    }}
   

通过上述实现，可以清晰地看到不同遍历方式在非递归情况下的实现差异。

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