Repository: vaskoz/dailycodingproblem-go (go)

Issue:
Day 125
Good morning! Here's your coding interview problem for today.

This problem was asked by Google.

Given the root of a binary search tree, and a target K, return two nodes in the tree whose sum equals K.

For example, given the following tree and K of 20
```
    10
   /   \
 5      15
       /  \
     11    15
```
Return the nodes 5 and 15.

Pull request:
Day125
Resolves #257.

Patch:
diff --git a/README.md b/README.md
index 170fce6..480dcba 100644
--- a/README.md
+++ b/README.md
@@ -130,3 +130,4 @@ problems from
 * [Day 120](https://github.com/vaskoz/dailycodingproblem-go/issues/248)
 * [Day 123](https://github.com/vaskoz/dailycodingproblem-go/issues/253)
 * [Day 124](https://github.com/vaskoz/dailycodingproblem-go/issues/255)
+* [Day 125](https://github.com/vaskoz/dailycodingproblem-go/issues/257)
diff --git a/day125/problem.go b/day125/problem.go
new file mode 100644
index 0000000..dba6a4b
--- /dev/null
+++ b/day125/problem.go
@@ -0,0 +1,45 @@
+package day125
+
+// BST is an integer binary search tree.
+type BST struct {
+	Val         int
+	Left, Right *BST
+}
+
+// FindTwoSumKInBST returns a slice of integer nodes that sum to K.
+// If no two nodes exist that sum to K, then return nil.
+// Runs in O(N log N) where N is the size of the BST.
+func FindTwoSumKInBST(head *BST, k int) []int {
+	return inorder(head, head, k, []int{})
+}
+
+func inorder(head, current *BST, k int, result []int) []int {
+	if current == nil {
+		return result
+	}
+	leftResult := inorder(head, current.Left, k, result)
+	if len(leftResult) == 2 {
+		return leftResult
+	}
+	node := searchBST(head, k-current.Val)
+	if node != nil && node != current {
+		return []int{current.Val, node.Val}
+	}
+	rightResult := inorder(head, current.Right, k, result)
+	if len(rightResult) == 2 {
+		return rightResult
+	}
+	return result
+}
+
+func searchBST(head *BST, target int) *BST {
+	if head == nil {
+		return nil
+	}
+	if target == head.Val {
+		return head
+		// } else if target < head.Val { // this part of searchBST is unused due to the
+		// 	return searchBST(head.Left, target) // inorder traversal that calls this search.
+	}
+	return searchBST(head.Right, target)
+}


Test patch:
diff --git a/day125/problem_test.go b/day125/problem_test.go
new file mode 100644
index 0000000..2952ea6
--- /dev/null
+++ b/day125/problem_test.go
@@ -0,0 +1,42 @@
+package day125
+
+import (
+	"reflect"
+	"testing"
+)
+
+var testcases = []struct {
+	head     *BST
+	k        int
+	expected []int
+}{
+	{&BST{10, &BST{5, nil, nil}, &BST{
+		15, &BST{11, nil, nil}, &BST{15, nil, nil},
+	}}, 20, []int{5, 15}},
+	{&BST{10, &BST{5, nil, nil}, &BST{
+		15, &BST{11, nil, nil}, &BST{15, nil, nil},
+	}}, 100, []int{}},
+	{&BST{10, &BST{5, nil, nil}, &BST{
+		15, &BST{11, nil, nil}, &BST{15, nil, nil},
+	}}, 15, []int{5, 10}},
+	{&BST{10, &BST{5, nil, nil}, &BST{
+		15, &BST{11, nil, nil}, &BST{15, nil, nil},
+	}}, 26, []int{11, 15}},
+}
+
+func TestFindTwoSumKInBST(t *testing.T) {
+	t.Parallel()
+	for _, tc := range testcases {
+		if result := FindTwoSumKInBST(tc.head, tc.k); !reflect.DeepEqual(result, tc.expected) {
+			t.Errorf("Expected %v got %v", tc.expected, result)
+		}
+	}
+}
+
+func BenchmarkFindTwoSumInBST(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, tc := range testcases {
+			FindTwoSumKInBST(tc.head, tc.k)
+		}
+	}
+}