leetcode刷题，总结，记录，备忘 173

leetcode173Binary 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.

题意没弄明白，，百度了一下，原来就是中序遍历，，，那就很简单了，，，

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class BSTIterator {public:    BSTIterator(TreeNode *root) {        while (root)        {            st.push(root);            root = root->left;        }    }        /** @return whether we have a next smallest number */    bool hasNext() {        return !st.empty();    }    /** @return the next smallest number */    int next() {        TreeNode * temp = st.top();        st.pop();        if (temp->right)        {            TreeNode * t = temp->right;            while (t)            {                st.push(t);                t = t->left;            }        }                return temp->val;    }private:   stack<TreeNode*> st;};/** * Your BSTIterator will be called like this: * BSTIterator i = BSTIterator(root); * while (i.hasNext()) cout << i.next(); */