LeetCode 173. Binary Search Tree Iterator-优快云博客

本文链接：https://blog.youkuaiyun.com/qq_31361913/article/details/106965420

本文介绍两种实现二叉搜索树迭代器的方法，一种是通过先序遍历将节点值存储在列表中，另一种是使用栈进行受控递归，以高效地找到下一个最小值。这两种方法都能在不改变原始树结构的情况下，按升序访问所有元素。

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Approach 1 (Flattening the BST)

注意访问List和Array不同，需要用get()方法，代码如下：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class BSTIterator {
    List<Integer> nodesSorted;
    int index;
    
    public BSTIterator(TreeNode root) {
        nodesSorted = new ArrayList<>();
        index = 0;
        inOrder(root);
    }
    
    private void inOrder(TreeNode root){
        if(root == null){
            return;
        }
        inOrder(root.left);
        nodesSorted.add(root.val);
        inOrder(root.right);
    }
    
    /** @return the next smallest number */
    public int next() {
        return nodesSorted.get(index++);
    }
    
    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        return index < nodesSorted.size();
    }
}

/**
 * Your BSTIterator object will be instantiated and called as such:
 * BSTIterator obj = new BSTIterator(root);
 * int param_1 = obj.next();
 * boolean param_2 = obj.hasNext();
 */

Approach 2 (Controlled Recursion)

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class BSTIterator {
    Stack<TreeNode> stack;
    
    public BSTIterator(TreeNode root) {
        stack = new Stack<>();
        leftMost(root);
    }
    
    private void leftMost(TreeNode root){
        while(root != null){
            stack.push(root);
            root = root.left;
        }
    }
    /** @return the next smallest number */
    public int next() {
        TreeNode top = stack.pop();
        if(top.right != null){
            leftMost(top.right);
        }
        return top.val;
    }
    
    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        return !stack.isEmpty();
    }
}

/**
 * Your BSTIterator object will be instantiated and called as such:
 * BSTIterator obj = new BSTIterator(root);
 * int param_1 = obj.next();
 * boolean param_2 = obj.hasNext();
 */