[youngDaLee] Week 02 solutions

3sum/youngDaLee.go

@@ -0,0 +1,51 @@
+package youngDaLee
+
+import (
+	"fmt"
+	"slices"
+)
+
+func threeSum(nums []int) [][]int {
+	// sort nums
+	slices.Sort(nums)
+
+	// unique-ify nums
+	uniqueKeys := make(map[string]bool)
+
+	var result [][]int
+	left := 0
+	for left < len(nums)-2 {
+		// init pivot, right
+		pivot := left + 1
+		right := len(nums) - 1
+
+		for pivot < right {
+			sum := nums[left] + nums[pivot] + nums[right]
+			unikeyKey := fmt.Sprintf("%d%d%d", nums[left], nums[pivot], nums[right])
+			if sum == 0 {
+				if _, ok := uniqueKeys[unikeyKey]; !ok {
+					uniqueKeys[unikeyKey] = true
+					result = append(result, []int{nums[left], nums[pivot], nums[right]})
+				}
+				for pivot < right-1 && nums[pivot] == nums[pivot+1] {
+					pivot += 1
+				}
+				for pivot < right-1 && nums[right] == nums[right-1] {
+					right -= 1
+				}
+				pivot += 1
+				right -= 1
+			} else if sum < 0 {
+				pivot += 1
+			} else if sum > 0 {
+				right -= 1
+			}
+		}
+
+		left += 1
+		for left < len(nums)-2 && nums[left] == nums[left-1] {
+			left += 1
+		}
+	}
+	return result
+}

validate-binary-search-tree/youngDaLee.go

@@ -0,0 +1,24 @@
+package youngDaLee
+
+import "math"
+
+type TreeNode struct {
+	Val   int
+	Left  *TreeNode
+	Right *TreeNode
+}
+
+func isValidBST(root *TreeNode) bool {
+	return dfs(root, math.MinInt, math.MaxInt)
+}
+
+func dfs(node *TreeNode, min, max int) bool {
+	if node == nil {
+		return true
+	}
+	if (min < node.Val && node.Val < max) == false {
+		return false
+	}
+
+	return dfs(node.Left, min, node.Val) && dfs(node.Right, node.Val, max)
+}