二叉树的非递归遍历总结

二叉树的非递归遍历总结

/** * * *非递归遍历 * * */
 
public class BinaryTree{
	//先序遍历，且为根左右
	public void preOrder(Node root){
		if(root == null)
			return;
		Stack<Node> stack = new Stack<>();
		stack.push(root);
		while(!stack.isEmpty()){
			Node cur = stack.pop();
			//先处理根节点
			System.out.println(cur.Value);
			//后处理左右子节点，由于是根左右的顺序，因此先处理左节点，要先入栈右节点
			if(cur.rightChild != null)
				stack.push(cur.rightChild);
			if(cur.leftChild != null)
				stack.push(cur.leftChild);
		}
	}
	//中序遍历，且为左根右
	public void midOrder(Node root){
		if(root == null)
			return;
		Stack<Node> stack = new Stack<>();
		Node myRoot = root;
		while(!stack.isEmpty() || myRoot != null){
			if(myRoot != null){
				stack.push(myRoot);
				myRoot = myRoot.leftChild;
			}else{
				myRoot = stack.pop();
				System.out.println(myRoot.value);
				myRoot = myRoot.rightChild;
			}
		}
	}
	//后续遍历，且为左右根
	//两个栈实现
	//栈2保存最后可以直接按顺序输出的值
	//栈1作为中间容器，实现栈2按顺序保存树的值
	public void posOrder(Node root){
		if(root == null)
			return;
		Stack<Node> stackOne = new Stack<>();
		Stack<Node> stackTwo = new Stack<>();
		stackOne.push(root);
		while(!stackOne.isEmpty()){
			Node cur = stackOne.pop();
			stackTwo.push(cur);
			if(cur.leftChild != null)
				stackOne.push(cur.leftChild);
			if(cur.rightChild != null)
				stackOne.push(cur.rightChild);
		}
		while(!stackTwo.isEmpty()){
			Node cur = stackTwo.pop();
			System.out.println(cur.value);
		}
	}
	//一个栈实现后续遍历
	//栈中保存一个子树的左子节点和根节点，这样可以先处理左子节点然后通过根节点处理右子节点，最后处理根节点
	public void posOrder2(Node root){
		if(root == null)
			return;
		Stack<Node> stack = new Stack<>();
		Node curNode = root;
		Node lastNode = null;
		while(curNode != null){
			stack.push(curNode);
			curNode = curNode.leftChild;
		}
		while(!stack.isEmpty()){
			curNode = stack.pop();
			//有右子树并且右子树不是上一次刚处理过，则不处理当前的根节点，当前根节点需要入栈，要先处理右子树才可以处理当前根节点
			//否则可以处理当前根节点
			if(curNode.rightChild != null && curNode.rightChild != lastNode){
				stack.push(curNode);
				curNode = curNode.rightChild;
				//因为是处理右子树，故仍需要将该右子树的左节点入栈
				while(curNode != null){
					stack.push(curNode);
					curNode = curNode.leftChild;
				}
			}else{
				//处理当前根节点
				System.out.println(curNode.value);
				lastNode = curNode;
			}
		}
	}
}
class Node{
    public int value;
    public Node leftChild;
    public Node rightChild;

    public Node(int value){
        this(value, null, null);
    }
    public Node(int value, Node leftChild, Node rightChild){
        this.value = value;
        this.leftChild = leftChild;
        this.rightChild = rightChild;
    }
}