LIKE CODING: LeetCode [173] Binary Search Tree Iterator

Tuesday, February 10, 2015

LeetCode [173] Binary Search Tree Iterator

173. Binary Search Tree Iterator

Medium

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.

Example:

BSTIterator iterator = new BSTIterator(root);
iterator.next();    // return 3
iterator.next();    // return 7
iterator.hasNext(); // return true
iterator.next();    // return 9
iterator.hasNext(); // return true
iterator.next();    // return 15
iterator.hasNext(); // return true
iterator.next();    // return 20
iterator.hasNext(); // return false

Note:

next() and hasNext() should run in average O(1) time and uses O(h) memory, where h is the height of the tree.
You may assume that next() call will always be valid, that is, there will be at least a next smallest number in the BST when next() is called.

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

    /** @return whether we have a next smallest number */
    bool hasNext() {
        return !stk.empty();
    }

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

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

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

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class BSTIterator {
    Stack<Integer> stk = new Stack();
    public void build(TreeNode node){
        if(node==null) return;
        build(node.right);
        stk.push(node.val);
        build(node.left);
    }
    public BSTIterator(TreeNode root) {
        build(root);
    }
    
    /** @return the next smallest number */
    public int next() {
        return stk.pop();
    }
    
    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        return !stk.isEmpty();
    }
}

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

LIKE CODING

Tuesday, February 10, 2015

LeetCode [173] Binary Search Tree Iterator

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