leetcode-173-Binary-Search-Tree-Iterator

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描述

Implement an iterator over a binary search tree (BST). Your iterator will be initialized with the root node of a BST.

Calling next() will return the next smallest number in the BST.

Note: next() and hasNext() should run in average O(1) time and uses O(h) memory, where h is the height of the tree.

分析

实现二叉搜索树，实际上就是写一个二叉树的中序遍历迭代器。

解决方案1(C++)

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/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class BSTIterator { 
    stack<TreeNode *> tree_stack;
public:
    BSTIterator(TreeNode *root) {
        while(root != NULL) {
            tree_stack.push(root);
            root = root->left;
        }
    }

    /** @return whether we have a next smallest number */
    bool hasNext() {
        return !tree_stack.empty();
    }

    /** @return the next smallest number */
    int next() {
        TreeNode* node = tree_stack.top();
        tree_stack.pop();
        int result = node->val;
        
        if(node->right != NULL) {
            node = node->right;
            while(node != NULL) {
                tree_stack.push(node);
                node = node->left;
            }
        }
        return result;
    }
};

/**
 * Your BSTIterator will be called like this:
 * BSTIterator i = BSTIterator(root);
 * while (i.hasNext()) cout << i.next();
 */

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