173.js

//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. Th
//e root of the BST is given as part of the constructor. The pointer should be ini
//tialized 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 t
//he right of the pointer, otherwise returns false.
// int next() Moves the pointer to the right, then returns the number at the poi
//nter.
//
//
// Notice that by initializing the pointer to a non-existent smallest number, th
//e 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.
//
//
//
// Follow up:
//
//
// Could you implement next() and hasNext() to run in average O(1) time and use
//O(h) memory, where h is the height of the tree?
//
// Related Topics 栈 树 设计
// 👍 444 👎 0


//leetcode submit region begin(Prohibit modification and deletion)
/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 */
var BSTIterator = function (root) {
  const stack = [];

  function traverse(n) {
    if (n.left) {
      traverse(n.left);
    }
    stack.push(n);

    if (n.right) {
      traverse(n.right);
    }
  }

  traverse(root);
  this.stack = stack;
};

/**
 * @return {number}
 */
BSTIterator.prototype.next = function () {
  return this.stack.shift().val;
};

/**
 * @return {boolean}
 */
BSTIterator.prototype.hasNext = function () {
  return !!this.stack.length;
};

/**
 * Your BSTIterator object will be instantiated and called as such:
 * var obj = new BSTIterator(root)
 * var param_1 = obj.next()
 * var param_2 = obj.hasNext()
 */
//leetcode submit region end(Prohibit modification and deletion)