LeetCode 173. Binary Search Tree Iterator（搜索二叉树）

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class BSTIterator {
public:
    vector<TreeNode*> stack;
    BSTIterator(TreeNode* root) {
        
        if(root!=NULL)
            DFS(root);
        
    }
    
    /** @return the next smallest number */
    int next() {
        
        TreeNode* term = stack[stack.size()-1];
        
        stack.pop_back();
        
        if(term->right!=NULL)
        {
            DFS(term->right);
        }
        
        return term->val;
        
    }
    
    /** @return whether we have a next smallest number */
    bool hasNext() {
        
        if(stack.size()!=0)
            return true;
        else
            return false;
        
    }
    
    void DFS(TreeNode* term)
    {
        
        stack.push_back(term);
        if(term->left!=NULL)
            DFS(term->left);
    }
};

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