Binary Search Tree Iterator 173

题目描述：

对于一个二叉查找树，设计一个迭代器，每次调用会返回下一个最小值

 

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题目分析：

没什么好说的二叉树的先序遍历

 

 

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代码：

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

    /** @return whether we have a next smallest number */
    bool hasNext() {
        return p;
    }

    /** @return the next smallest number */
    int next() {
        int ret=p->val;
        if(p->right){
            toLeft(p->right);
        }else {
            if(stk.empty())p=NULL;
            else {
                p=stk.top();
                stk.pop();
                //toLeft(p->right);
            }
        }
        return ret;
    }
};

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