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import java.util.Stack;public class PreOrder {    public static class Node {        public int value;        public Node left;        public Node right;        public Node(int value) {            this.value = value;        }    }    //前序遍历    public static void preOrder(Node node) {        Stack
   
     stack = new Stack<>();        stack.push(node);        while (!stack.isEmpty()) {            node = stack.pop();            System.out.print(node.value + " ");            if (node.right != null) {                stack.push(node.right);            }            if (node.left != null) {                stack.push(node.left);            }        }    }    //中序遍历    public static void inOrder(Node node) {        Stack
    
      stack = new Stack<>();        while (!stack.isEmpty() || node != null) {            if (node != null) {                stack.push(node);                node = node.left;            } else {                node = stack.pop();                System.out.print(node.value + " ");                node = node.right;            }        }    }    //后序遍历    public static void postOrder(Node node) {    	// 辅助栈，类似于前序遍历压入节点，最后反转即可        Stack
     
       help = new Stack<>();        Stack
      
        stack = new Stack<>();        stack.push(node);        while (!stack.isEmpty()) {            node = stack.pop();            help.push(node);            //这里和前序遍历不同，先将左节点压入栈，再压右节点            if (node.left != null) {                stack.push(node.left);            }            if (node.right != null) {                stack.push(node.right);            }        }        //将栈反转即得到后序遍历的结果        while (!help.isEmpty()) {            System.out.print(help.pop().value + " ");        }    }    public static void main(String[] args) {        Node head = new Node(1);        head.left = new Node(2);        head.right = new Node(5);        head.left.left = new Node(3);        head.left.right = new Node(4);        head.right.left = new Node(6);        head.right.right = new Node(7);//        preOrder(head);//1 2 3 4 5 6 7        inOrder(head);//3 2 4 1 6 5 7//        postOrder(head);//3 4 2 6 7 5 1    }}
      
     
    
   

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