主要利用栈的后进先出的特性
public class TraversalNonRecursive {
/**
* 先压头节点并弹出，然后先压右，后压左。这样弹出的顺序就是先左后右
* @param head
*/
public static void preOrder(TreeNode head){
System.out.println("preOrder");
if(head!=null){
Stack<TreeNode> stack = new Stack<>();
stack.push(head);
while (!stack.isEmpty()){
head=stack.pop();
System.out.print(head.val+" ");
if(head.right!=null){
stack.push(head.right);
}
if(head.left!=null){
stack.push(head.left);
}
}
}
}
/**
* 将二叉树左边界依次入栈，直到为空，弹出，并使head等于右节点，此时如果head不为空，则压栈，为空则弹出。弹出的顺序刚好是中序遍历
* @param head
*/
public static void inOrder(TreeNode head){
System.out.println("inOrder");
if(head!=null){
Stack<TreeNode> stack = new Stack<>();
while (!stack.isEmpty()||head!=null){
if(head!=null){
stack.push(head);
head=head.left;
}else {
head=stack.pop();
System.out.print(head.val+" ");
head=head.right;
}
}
}
}
/**
* 1.后序遍历顺序是左右中，而中右左的遍历很容易构造，跟先序遍历差不多，只是左右节点压栈顺序调换
* 2.将中右左遍历压入一个辅助栈，再弹出，也就是后序的左右中遍历
* @param head
*/
public static void posOrder(TreeNode head){
System.out.println("posOrder");
if(head!=null){
Stack<TreeNode> stack = new Stack<>();
Stack<TreeNode> stackHelp = new Stack<>();
stack.push(head);
while (!stack.isEmpty()){
head=stack.pop();
stackHelp.push(head);
if(head.left!=null){
stack.push(head.left);
}
if(head.right!=null){
stack.push(head.right);
}
}
while (!stackHelp.isEmpty()){
System.out.print(stackHelp.pop().val+" ");
}
}
}
public static void main(String[] args) {
TreeNode treeNode1=new TreeNode(1);
TreeNode treeNode2=new TreeNode(2);
TreeNode treeNode3=new TreeNode(3);
TreeNode treeNode4=new TreeNode(4);
TreeNode treeNode5=new TreeNode(5);
TreeNode treeNode6=new TreeNode(6);
TreeNode treeNode7=new TreeNode(7);
treeNode1.left=treeNode2;
treeNode1.right=treeNode3;
treeNode1.left.left=treeNode4;
treeNode1.left.right=treeNode5;
treeNode1.right.left=treeNode6;
treeNode1.right.right=treeNode7;
preOrder(treeNode1);
System.out.println("\n");
inOrder(treeNode1);
System.out.println("\n");
posOrder(treeNode1);
}
}
示例二叉树
打印结果