Algorithm

Problem Name: 173. Binary Search Tree Iterator

Problem Link: https://leetcode.com/problems/binary-search-tree-iterator/
Implement the BSTIterator class that represents an iterator over the in-order traversal of a binary search tree (BST):
  • BSTIterator(TreeNode root) Initializes an object of the BSTIterator class. The root of the BST is given as part of the constructor. The pointer should be initialized to a non-existent number smaller than any element in the BST.
  • boolean hasNext() Returns true if there exists a number in the traversal to the right of the pointer, otherwise returns false.
  • int next() Moves the pointer to the right, then returns the number at the pointer.

Notice that by initializing the pointer to a non-existent smallest number, the first call to next() will return the smallest element in the BST.

You may assume that next() calls will always be valid. That is, there will be at least a next number in the in-order traversal when next() is called.

 

Example 1:

Input
["BSTIterator", "next", "next", "hasNext", "next", "hasNext", "next", "hasNext", "next", "hasNext"]
[[[7, 3, 15, null, null, 9, 20]], [], [], [], [], [], [], [], [], []]
Output
[null, 3, 7, true, 9, true, 15, true, 20, false]

Explanation
BSTIterator bSTIterator = new BSTIterator([7, 3, 15, null, null, 9, 20]);
bSTIterator.next();    // return 3
bSTIterator.next();    // return 7
bSTIterator.hasNext(); // return True
bSTIterator.next();    // return 9
bSTIterator.hasNext(); // return True
bSTIterator.next();    // return 15
bSTIterator.hasNext(); // return True
bSTIterator.next();    // return 20
bSTIterator.hasNext(); // return False

 

Constraints:

  • The number of nodes in the tree is in the range [1, 105].
  • 0 <= Node.val <= 106
  • At most 105 calls will be made to hasNext, and next.

Code Examples

#1 Code Example with C Programming

Code - C Programming


struct BSTIterator {
    struct TreeNode **stack;
    int sp;
};
​
int depth(struct TreeNode *node) {
    int l, r;
    if (!node) return 0;
    l = depth(node->left) + 1;
    r = depth(node->right) + 1;
    if (l > r) return l;
    return r;
}
void push_left(struct BSTIterator *obj, struct TreeNode *node) {
    do {
        obj->stack[obj->sp ++] = node;
        node = node->left;
    } while (node);
}
struct BSTIterator *bstIteratorCreate(struct TreeNode *root) {
    struct BSTIterator *obj;
    int d = depth(root);
    
    obj = calloc(1, sizeof(*obj));
    //assert(obj);
    if (d) {
        obj->stack = malloc(d * sizeof(struct TreeNode *));
        //assert(obj->stack);
        push_left(obj, root);
    }
    
    return obj;
}
​
/** @return whether we have a next smallest number */
bool bstIteratorHasNext(struct BSTIterator *iter) {
    return (iter->sp) ? true : false;
}
​
/** @return the next smallest number */
int bstIteratorNext(struct BSTIterator *iter) {
    struct TreeNode *node;
    
    node = iter->stack[-- iter->sp];
    if (node->right) push_left(iter, node->right);
    
    return node->val;
}
​
/** Deallocates memory previously allocated for the iterator */
void bstIteratorFree(struct BSTIterator *iter) {
    if (iter->stack) free(iter->stack);
    free(iter);
}
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Input

x
–
+
cmd
["BSTIterator", "next", "next", "hasNext", "next", "hasNext", "next", "hasNext", "next", "hasNext"] [[[7, 3, 15, null, null, 9, 20]], [], [], [], [], [], [], [], [], []]

Output

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cmd
[null, 3, 7, true, 9, true, 15, true, 20, false]

#2 Code Example with C++ Programming

Code - C++ Programming


class BSTIterator {
public:
    BSTIterator(TreeNode *root) {
        load(s, root);
    }

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

    /** @return the next smallest number */
    int next() {
        int n = s.top();
        s.pop();
        return n;
    }
    
private:
    stack < int>s;
    
    void load(stack<int>& s, TreeNode* root){
        if(!root) return;
        load(s, root->right);
        s.push(root->val);
        load(s, root->left);
    }
};
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Input

x
–
+
cmd
["BSTIterator", "next", "next", "hasNext", "next", "hasNext", "next", "hasNext", "next", "hasNext"] [[[7, 3, 15, null, null, 9, 20]], [], [], [], [], [], [], [], [], []]

Output

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cmd
[null, 3, 7, true, 9, true, 15, true, 20, false]

#4 Code Example with Javascript Programming

Code - Javascript Programming


const BSTIterator = function(root) {
  this.root = root;
  this.stack = [];
};

/**
 * @return the next smallest number
 * @return {number}
 */
BSTIterator.prototype.next = function() {
  while (this.root) {
    this.stack.push(this.root);
    this.root = this.root.left;
  }
  this.root = this.stack.pop();
  const result = this.root.val;
  this.root = this.root.right;
  return result;
};

/**
 * @return whether we have a next smallest number
 * @return {boolean}
 */
BSTIterator.prototype.hasNext = function() {
  return this.root || this.stack.length;
};
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Input

x
–
+
cmd
["BSTIterator", "next", "next", "hasNext", "next", "hasNext", "next", "hasNext", "next", "hasNext"] [[[7, 3, 15, null, null, 9, 20]], [], [], [], [], [], [], [], [], []]

Output

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cmd
[null, 3, 7, true, 9, true, 15, true, 20, false]

#5 Code Example with Python Programming

Code - Python Programming


class BSTIterator:
    def __init__(self, root: TreeNode):
        self.stack = []
        self.pushAll(root)

    def next(self) -> int:
        cur = self.stack.pop()
        self.pushAll(cur.right)
        return cur.val

    def hasNext(self) -> bool:
        return self.stack

    def pushAll(self, node):
        while node != None:
            self.stack += (node,)
            node = node.left
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Input

x
–
+
cmd
["BSTIterator", "next", "next", "hasNext", "next", "hasNext", "next", "hasNext", "next", "hasNext"] [[[7, 3, 15, null, null, 9, 20]], [], [], [], [], [], [], [], [], []]

Output

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cmd
[null, 3, 7, true, 9, true, 15, true, 20, false]

#6 Code Example with C# Programming

Code - C# Programming


using System.Collections.Generic;

namespace LeetCode
{
    public class _0173_BinarySearchTreeIterator
    {
        private readonly Stack < TreeNode> stack;

        public _0173_BinarySearchTreeIterator(TreeNode root)
        {
            stack = new Stack();
            LeftMostInorder(root);
        }

        private void LeftMostInorder(TreeNode root)
        {
            while (root != null)
            {
                stack.Push(root);
                root = root.left;
            }
        }

        /** @return the next smallest number */
        public int Next()
        {
            var node = stack.Pop();

            if (node.right != null)
                LeftMostInorder(node.right);

            return node.val;
        }

        /** @return whether we have a next smallest number */
        public bool HasNext()
        {
            return stack.Count > 0;
        }
    }
}
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Input

x
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+
cmd
["BSTIterator", "next", "next", "hasNext", "next", "hasNext", "next", "hasNext", "next", "hasNext"] [[[7, 3, 15, null, null, 9, 20]], [], [], [], [], [], [], [], [], []]

Output

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cmd
[null, 3, 7, true, 9, true, 15, true, 20, false]
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