LeetCode-JAVA-Binary Search Tree Iterator1._java binarysearch sourcecode-优快云博客

本文链接：https://blog.youkuaiyun.com/clevery017/article/details/51986499

本文介绍两种实现二叉搜索树迭代器的方法：一种通过中序遍历将节点值存入链表；另一种利用栈特性按需压入左子树节点并返回最小元素。

1. 不考虑空间复杂度，将二叉树中序遍历，插入到LinkedList中。

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

public class BSTIterator {
    
    private List<Integer> list = new LinkedList<Integer>();

    public BSTIterator(TreeNode root) {
        midSearch(root);
    }
    
    private void midSearch(TreeNode root){
        if(root == null){
            return;
        }
        midSearch(root.left);
        list.add(root.val);
        midSearch(root.right);
    }

    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        if(list==null || list.size()==0){
            return false;
        }else{
            return true;
        }
    }

    /** @return the next smallest number */
    public int next() {
        int tmp = this.list.get(0);
        list.remove(0);
        return tmp;
    }
}

/**
 * Your BSTIterator will be called like this:
 * BSTIterator i = new BSTIterator(root);
 * while (i.hasNext()) v[f()] = i.next();
 */

2. 从根到左树压入stack

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

public class BSTIterator {
    
    private Stack<TreeNode> stack = new Stack<TreeNode>();
    private TreeNode root;
    private int outValue;

    public BSTIterator(TreeNode root) {
        this.root = root;
        pushLeft(root);
    }
    
    private void pushLeft(TreeNode root){
        if(root == null){
            return;
        }
        stack.push(root);
        pushLeft(root.left);
    }

    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        if(stack.size()<=0){
            return false;
        }else{
            TreeNode root = stack.pop();
            outValue = root.val;
            root = root.right;
            while(root!=null){
                stack.push(root);
                root = root.left;
            }
            return true;
        }
    }

    /** @return the next smallest number */
    public int next() {
        return outValue;
    }
}

/**
 * Your BSTIterator will be called like this:
 * BSTIterator i = new BSTIterator(root);
 * while (i.hasNext()) v[f()] = i.next();
 */