173 added

Author: aQuaYi

Algorithms/0173.binary-search-tree-iterator/README.md

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+# [173. Binary Search Tree Iterator](https://leetcode.com/problems/binary-search-tree-iterator/)
+
+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.
+
+![bst](bst.png)
+
+Example:
+
+```text
+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.

Algorithms/0173.binary-search-tree-iterator/binary-search-tree-iterator.go

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+package problem0173
+
+import "github.com/aQuaYi/LeetCode-in-Go/kit"
+
+// TreeNode is pre-defined...
+/* Definition for a binary tree node.
+ * type TreeNode struct {
+ *     Val int
+ *     Left *TreeNode
+ *     Right *TreeNode
+ * }
+ */
+type TreeNode = kit.TreeNode
+
+// BSTIterator is the iterator of BST
+type BSTIterator struct {
+}
+
+// Constructor returns a BST iterator
+func Constructor(root *TreeNode) BSTIterator {
+
+	return BSTIterator{}
+}
+
+// Next returns the next smallest number */
+func (it *BSTIterator) Next() int {
+
+	return 0
+}
+
+// HasNext returns whether we have a next smallest number */
+func (it *BSTIterator) HasNext() bool {
+
+	return false
+}

Algorithms/0173.binary-search-tree-iterator/binary-search-tree-iterator_test.go

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+package problem0173
+
+import (
+	"testing"
+
+	"github.com/aQuaYi/LeetCode-in-Go/kit"
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_BSTIterator(t *testing.T) {
+	ast := assert.New(t)
+
+	ints := []int{7, 3, 15, kit.NULL, kit.NULL, 9, 20}
+
+	root := kit.Ints2TreeNode(ints)
+
+	it := Constructor(root)
+
+	i := 0
+
+	for it.HasNext() {
+		for ints[i] == kit.NULL {
+			i++
+		}
+		ast.Equal(ints[i], it.Next(), "%d", ints[i])
+	}
+}