[LeetCode]Binary Search Tree Iterator

Binary Search Tree Iterator

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.

Credits:
Special thanks to @ts for adding this problem and creating all test cases.

题意差不多就是将BST按中序非递归输出。当然stack。

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

    /** @return whether we have a next smallest number */
    bool hasNext() {
        if(s.empty()) return false;
        else return true;
    }

    /** @return the next smallest number */
    int next() {
        TreeNode* tmp = s.top();
        int result = tmp->val;
        s.pop();
        tmp = tmp->right;  
        while(tmp)  
        {  
            s.push(tmp);  
            tmp = tmp->left;  
        }  
        return result;
    }
};

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