Leetcode 173. Binary Search Tree Iterator

文章作者：Tyan
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1. Description

2. Solution

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class BSTIterator {
private:
    stack<TreeNode*> index;
public:
    BSTIterator(TreeNode *root) {
        traverseLeft(root);
    }

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

    /** @return the next smallest number */
    int next() {
        TreeNode* current = index.top();
        index.pop();
        traverseLeft(current->right);
        return current->val;
    }
    
    void traverseLeft(TreeNode* root) {
        TreeNode* current = root;
        while(current) {
            index.push(current);
            current = current->left;
        }
    }
};

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

Reference

1. https://leetcode.com/problems/binary-search-tree-iterator/description/