diff --git a/HamSungJun/HashTable/Solution_594.ts b/HamSungJun/HashTable/Solution_594.ts
new file mode 100644
index 0000000..afaf267
--- /dev/null
+++ b/HamSungJun/HashTable/Solution_594.ts
@@ -0,0 +1,23 @@
+function findLHS (nums: number[]): number {
+  const countMap = new Map()
+  nums.forEach(num => {
+    if (countMap.has(num)) {
+      countMap.set(num, countMap.get(num) + 1)
+    } else {
+      countMap.set(num, 1)
+    }
+  })
+  const mapToArray = [...countMap.entries()]
+  mapToArray.sort((a, b) => {
+    return Number(a[0] > b[0]) - Number(a[0] < b[0])
+  })
+  let maxLHS = Number.MIN_SAFE_INTEGER
+  for (let i = 0; i < mapToArray.length - 1; i++) {
+    if (mapToArray[i + 1][0] - mapToArray[i][0] === 1) {
+      maxLHS = Math.max(maxLHS, mapToArray[i + 1][1] + mapToArray[i][1])
+    }
+  }
+  return maxLHS
+};
+
+findLHS([1, 3, 2, 2, 5, 2, 3, 7])
diff --git a/HamSungJun/HashTable/Solution_645.ts b/HamSungJun/HashTable/Solution_645.ts
new file mode 100644
index 0000000..3c36704
--- /dev/null
+++ b/HamSungJun/HashTable/Solution_645.ts
@@ -0,0 +1,14 @@
+function findErrorNums (nums: number[]): number[] {
+  const countArray = new Array(nums.length + 1).fill(0)
+  const answer = []
+  nums.forEach(num => {
+    countArray[num] += 1
+    if (countArray[num] === 2) {
+      answer.push(num)
+    }
+  })
+  answer.push(countArray.findIndex((v, i) => v === 0 && i !== 0))
+  return answer
+};
+
+console.log(findErrorNums([1, 1]))
diff --git a/HamSungJun/HashTable/Solution_720.ts b/HamSungJun/HashTable/Solution_720.ts
new file mode 100644
index 0000000..5c0b929
--- /dev/null
+++ b/HamSungJun/HashTable/Solution_720.ts
@@ -0,0 +1,44 @@
+function longestWord (words: string[]): string {
+  const wordSet = new Set()
+  const lengthMap:Map<number, string[]> = new Map()
+  let possibleAnswers = []
+  let longestAnswers = []
+  for (let i = 0; i < words.length; i++) {
+    const nextWord = words[i]
+    const wordLen = nextWord.length
+    if (!wordSet.has(nextWord)) {
+      wordSet.add(nextWord)
+      if (lengthMap.has(wordLen)) {
+        (lengthMap.get(wordLen) as string[]).push(nextWord)
+      } else {
+        lengthMap.set(wordLen, [nextWord])
+      }
+    }
+  }
+  if (!lengthMap.has(1)) {
+    return ''
+  }
+  possibleAnswers = [...(lengthMap.get(1) as string[])]
+  longestAnswers = possibleAnswers.slice()
+  while (possibleAnswers.length !== 0) {
+    const nextItem = possibleAnswers.shift() as string
+    if (lengthMap.has(nextItem.length + 1)) {
+      const reachableList = lengthMap.get(nextItem.length + 1)?.filter(str => str.startsWith(nextItem)) as string[]
+      possibleAnswers.push(...reachableList)
+      longestAnswers.push(...reachableList)
+    }
+  }
+  const maxLongestWord = longestAnswers[longestAnswers.length - 1].length
+  possibleAnswers = []
+  for (let i = longestAnswers.length - 1; i >= 0; i--) {
+    if (longestAnswers[i].length === maxLongestWord) {
+      possibleAnswers.push(longestAnswers[i])
+    } else {
+      break
+    }
+  }
+  possibleAnswers.sort()
+  return possibleAnswers[0]
+};
+
+console.log(longestWord(['a', 'banana', 'app', 'appl', 'ap', 'apply', 'apple']))