diff --git a/binary-tree-maximum-path-sum/yhkee0404.go b/binary-tree-maximum-path-sum/yhkee0404.go
new file mode 100644
index 000000000..ea1221f8b
--- /dev/null
+++ b/binary-tree-maximum-path-sum/yhkee0404.go
@@ -0,0 +1,23 @@
+/**
+ * Definition for a binary tree node.
+ * type TreeNode struct {
+ *     Val int
+ *     Left *TreeNode
+ *     Right *TreeNode
+ * }
+ */
+func maxPathSum(root *TreeNode) int {
+    ans, _ := solve(root)
+    return ans
+}
+
+func solve(root *TreeNode) (int, int) {
+    if root == nil {
+        return -30_000_000, -30_000_000
+    }
+    leftAns, leftRootAns := solve(root.Left)
+    rightAns, rightRootAns := solve(root.Right)
+    rootAns := root.Val + max(0, leftRootAns, rightRootAns,)
+    ans := max(leftAns, rightAns, root.Val + max(0, leftRootAns,) + max(0, rightRootAns,))
+    return ans, rootAns
+}
diff --git a/graph-valid-tree/yhkee0404.kt b/graph-valid-tree/yhkee0404.kt
new file mode 100644
index 000000000..7dbb7ca88
--- /dev/null
+++ b/graph-valid-tree/yhkee0404.kt
@@ -0,0 +1,48 @@
+class Solution {
+    /**
+     * @param n: An integer
+     * @param edges: a list of undirected edges
+     * @return: true if it's a valid tree, or false
+     */
+    fun validTree(n: Int, edges: Array<IntArray>): Boolean {
+        // write your code here
+        val adj = List(n) {mutableListOf<Int>()} // S(V, E) = O(V + E)
+        edges.forEach {
+                    adj[it[0]].add(it[1])
+                    adj[it[1]].add(it[0])
+                }
+        var t = 0
+        val tortoiseStack = mutableListOf<MutableList<Int>>(mutableListOf(0, 0, -1))
+        val hareStack = mutableListOf<MutableList<Int>>(mutableListOf(0, 0, -1))
+        var tortoise = listOf(-1)
+        while (! hareStack.isEmpty()) { // T(V, E) = O(V + E)
+            val hare = hareStack.last()
+            if (hare[1] == adj[hare[0]].size) {
+                hareStack.removeLast()
+            } else if (adj[hare[0]][hare[1]] != hare[2]) {
+                hareStack.add(mutableListOf(adj[hare[0]][hare[1]], 0, hare[0]))
+            }
+            if (hare[1]++ != 0) {
+                continue
+            }
+            if ((t++ and 1) == 1) {
+                while (! tortoiseStack.isEmpty()) {
+                    tortoise = tortoiseStack.last()
+                    if (tortoise[1] == adj[tortoise[0]].size) {
+                        tortoiseStack.removeLast()
+                    } else if (adj[tortoise[0]][tortoise[1]] != tortoise[2]) {
+                        tortoiseStack.add(mutableListOf(adj[tortoise[0]][tortoise[1]], 0, tortoise[0]))
+                    }
+                    if (tortoise[1]++ != 0) {
+                        continue
+                    }
+                    break
+                }
+            }
+            if (hare[0] == tortoise[0]) {
+                return false
+            }
+        }
+        return t == n
+    }
+}
diff --git a/merge-intervals/yhkee0404.rs b/merge-intervals/yhkee0404.rs
new file mode 100644
index 000000000..b3d111514
--- /dev/null
+++ b/merge-intervals/yhkee0404.rs
@@ -0,0 +1,21 @@
+use itertools::Itertools;
+use std::cmp::max;
+
+impl Solution {
+    pub fn merge(intervals: Vec<Vec<i32>>) -> Vec<Vec<i32>> {
+        let mut intervals: Vec<_> = intervals.iter()
+                .cloned()
+                .sorted_by(|a, b| a.cmp(b)) // T(n) = O(nlogn)
+                .collect();
+        let mut ans: Vec<Vec<i32>> = vec![];
+        let mut end = -1;
+        for interval in intervals {
+            if ans.is_empty() || ans.last().unwrap()[1] < interval[0] {
+                ans.push(interval) // S(n) = O(n)
+            } else {
+                ans.last_mut().unwrap()[1] = max(ans.last().unwrap()[1], interval[1])
+            }
+        }
+        ans
+    }
+}
diff --git a/missing-number/yhkee0404.scala b/missing-number/yhkee0404.scala
new file mode 100644
index 000000000..8d828cc1a
--- /dev/null
+++ b/missing-number/yhkee0404.scala
@@ -0,0 +1,5 @@
+object Solution {
+    def missingNumber(nums: Array[Int]): Int = {
+        (1 to nums.length).reduce(_ ^ _) ^ nums.reduce(_ ^ _) // T(n) = O(n), S(n) = O(1)
+    }
+}
diff --git a/reorder-list/yhkee0404.swift b/reorder-list/yhkee0404.swift
new file mode 100644
index 000000000..ec8369797
--- /dev/null
+++ b/reorder-list/yhkee0404.swift
@@ -0,0 +1,37 @@
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     public var val: Int
+ *     public var next: ListNode?
+ *     public init() { self.val = 0; self.next = nil; }
+ *     public init(_ val: Int) { self.val = val; self.next = nil; }
+ *     public init(_ val: Int, _ next: ListNode?) { self.val = val; self.next = next; }
+ * }
+ */
+class Solution {
+    func reorderList(_ head: ListNode?) {
+        let temp = ListNode(0, head)
+        solve(temp, temp, false)
+    }
+    func solve(_ tortoise: ListNode, _ hare: ListNode?, _ odd: Bool) -> ListNode? {
+        guard let safeHare = hare else {
+            let tail = ListNode(0, tortoise.next)
+            tortoise.next = nil
+            return tail
+        }
+        let tail = solve(odd ? tortoise : tortoise.next!, safeHare.next, !odd)
+        if !odd {
+            return tail
+        }
+        guard let safeTail = tail else {
+            return nil
+        }
+        if safeTail.val == 0 {
+            return safeTail.next
+        }
+        let next = safeTail.next
+        safeTail.next = tortoise.next
+        tortoise.next = safeTail
+        return next
+    }
+}