leetcode每日一题-173. Binary Search Tree Iterator

1.先dfs，保存dfs的所有值的o(n)做法

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class BSTIterator {
public:
    int node[100003];
    int cnt=0;
    int idx=0;
    void dfs(TreeNode* u){
        if(u==nullptr) return;
        dfs(u->left);
        node[cnt++]=u->val;
        dfs(u->right);
    }
    
    BSTIterator(TreeNode* root) {
        memset(node,0,sizeof node);
        dfs(root); 
    }
    
    int next() {
        //dfs的下个结点
        return node[idx++];
    }
    
    bool hasNext() {
        //只要不是最后一个结点全为true
        if(idx!=cnt) return true;
        else return false;
    }
};

/**
 * 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();
 */

2.维护递归栈的做法 o(h)

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class BSTIterator {
private:
    TreeNode* cur;
    stack<TreeNode*> stk;
    
public:
    BSTIterator(TreeNode* root) {
        cur=root;
    }
    
    int next() {
        while(cur!=nullptr){//将cur左边的全部进栈
            stk.push(cur);
            cur=cur->left;
        }
        cur=stk.top();
        stk.pop();
        int ret=cur->val;
        cur=cur->right;
        return ret;
    }
    
    bool hasNext() {
        if(cur!=nullptr || !stk.empty()) return true;
        return false;
    }
};

/**
 * 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();
 */