diff --git a/src/problem1.js b/src/problem1.js
index 9a14f2c..1dff2cf 100644
--- a/src/problem1.js
+++ b/src/problem1.js
@@ -1,6 +1,35 @@
 function problem1(pobi, crong) {
-  var answer;
-  return answer;
+  if (!isValidPage(pobi) || !isValidPage(crong)) return -1;
+
+  const pScore = Math.max(pageScore(pobi[0]), pageScore(pobi[1]));
+  const cScore = Math.max(pageScore(crong[0]), pageScore(crong[1]));
+
+  if (pScore > cScore) return 1;
+  if (pScore < cScore) return 2;
+  return 0;
+}
+
+function isValidPage(arr) {
+  if (!Array.isArray(arr) || arr.length !== 2) return false;
+  const [L, R] = arr;
+  if (!Number.isInteger(L) || !Number.isInteger(R)) return false;
+
+  if (L < 1 || R > 400) return false;
+  if (L === 1 && R === 2) return false;
+  if (L === 399 && R === 400) return false;
+
+  if (L % 2 !== 1) return false;
+  if (R % 2 !== 0) return false;
+  if (R !== L + 1) return false;
+
+  return true;
+}
+
+function pageScore(n) {
+  const digits = String(n).split('').map(Number);
+  const sum = digits.reduce((a, b) => a + b, 0);
+  const product = digits.reduce((a, b) => a * b, 1);
+  return Math.max(sum, product);
 }
 
 module.exports = problem1;
diff --git a/src/problem2.js b/src/problem2.js
index cebd07c..b41a2f7 100644
--- a/src/problem2.js
+++ b/src/problem2.js
@@ -1,6 +1,21 @@
 function problem2(cryptogram) {
-  var answer;
-  return answer;
+  let changed = true;
+
+  while (changed) {
+    changed = false;
+    let result = "";
+    for (let i = 0; i < cryptogram.length; i++) {
+      if (cryptogram[i] === cryptogram[i + 1]) {
+        changed = true;
+        i++;
+      } else {
+        result += cryptogram[i];
+      }
+    }
+    cryptogram = result;
+  }
+
+  return cryptogram;
 }
 
-module.exports = problem2;
+module.exports = problem2;
\ No newline at end of file
diff --git a/src/problem3.js b/src/problem3.js
index 1baed28..62e565f 100644
--- a/src/problem3.js
+++ b/src/problem3.js
@@ -1,6 +1,16 @@
 function problem3(number) {
-  var answer;
+  let answer = 0;
+
+  for (let i = 1; i <= number; i++) {
+    const str = String(i);
+    for (let item of str) {
+      if (item === "3" || item === "6" || item === "9") {
+        answer++;
+      }
+    }
+  }
+
   return answer;
 }
 
-module.exports = problem3;
+module.exports = problem3;
\ No newline at end of file
diff --git a/src/problem4.js b/src/problem4.js
index ee1d3bd..556e89c 100644
--- a/src/problem4.js
+++ b/src/problem4.js
@@ -1,6 +1,17 @@
 function problem4(word) {
-  var answer;
-  return answer;
+  let result = '';
+
+  for (let item of word) {
+    if (item >= 'A' && item <= 'Z') {
+      result += String.fromCharCode('Z'.charCodeAt(0) - (item.charCodeAt(0) - 'A'.charCodeAt(0)));
+    } else if (item >= 'a' && item <= 'z') {
+      result += String.fromCharCode('z'.charCodeAt(0) - (item.charCodeAt(0) - 'a'.charCodeAt(0)));
+    } else {
+      result += item;
+    }
+  }
+
+  return result;
 }
 
 module.exports = problem4;
diff --git a/src/problem5.js b/src/problem5.js
index 9368e87..8ae649f 100644
--- a/src/problem5.js
+++ b/src/problem5.js
@@ -1,6 +1,14 @@
 function problem5(money) {
-  var answer;
-  return answer;
+  const units = [50000, 10000, 5000, 1000, 500, 100, 50, 10, 1];
+  const result = [];
+
+  for (let unit of units) {
+    const count = Math.floor(money / unit);
+    result.push(count);
+    money %= unit;
+  }
+  
+  return result;
 }
 
 module.exports = problem5;
diff --git a/src/problem6.js b/src/problem6.js
index 3f842b5..22b1f0a 100644
--- a/src/problem6.js
+++ b/src/problem6.js
@@ -1,6 +1,25 @@
 function problem6(forms) {
-  var answer;
-  return answer;
+  let badEmails = [];
+
+  for (let i = 0; i < forms.length; i++) {
+    const [email1, nickname1] = forms[i];
+
+    for (let j = i + 1; j < forms.length; j++) {
+      const [email2, nickname2] = forms[j];
+
+      for (let k = 0; k < nickname1.length - 1; k++) {
+        const part = nickname1.substring(k, k + 2);
+        if (nickname2.includes(part)) {
+          if (!badEmails.includes(email1)) badEmails.push(email1);
+          if (!badEmails.includes(email2)) badEmails.push(email2);
+          break;
+        }
+      }
+    }
+  }
+
+  badEmails.sort();
+  return badEmails;
 }
 
 module.exports = problem6;
diff --git a/src/problem7.js b/src/problem7.js
index ee1bb9d..07c9c50 100644
--- a/src/problem7.js
+++ b/src/problem7.js
@@ -1,6 +1,70 @@
 function problem7(user, friends, visitors) {
-  var answer;
-  return answer;
+  var graph = {};
+  for (var i = 0; i < friends.length; i++) {
+    var a = friends[i][0];
+    var b = friends[i][1];
+
+    if (!graph[a]) graph[a] = [];
+    if (!graph[b]) graph[b] = [];
+    graph[a].push(b);
+    graph[b].push(a);
+  }
+
+  var userFriends = [];
+  if (graph[user]) {
+    for (var i = 0; i < graph[user].length; i++) {
+      userFriends.push(graph[user][i]);
+    }
+  }
+  var score = {};
+
+  for (var i = 0; i < userFriends.length; i++) {
+    var friend = userFriends[i];
+    var fList = graph[friend] ? graph[friend] : [];
+
+    for (var j = 0; j < fList.length; j++) {
+      var person = fList[j];
+      if (person === user) continue;
+      var isUserFriend = false;
+      for (var k = 0; k < userFriends.length; k++) {
+        if (userFriends[k] === person) {
+          isUserFriend = true;
+          break;
+        }
+      }
+      if (isUserFriend) continue;
+      if (!score[person]) score[person] = 0;
+      score[person] += 10;
+    }
+  }
+
+  for (var i = 0; i < visitors.length; i++) {
+    var v = visitors[i];
+    if (v === user) continue;
+    var isUserFriend = false;
+    for (var k = 0; k < userFriends.length; k++) {
+      if (userFriends[k] === v) {
+        isUserFriend = true;
+        break;
+      }
+    }
+    if (isUserFriend) continue;
+    if (!score[v]) score[v] = 0;
+    score[v] += 1;
+  }
+
+  var keys = [];
+  for (var key in score) {
+    if (score[key] > 0) keys.push(key);
+  }
+  keys.sort(function (a, b) {
+    if (score[b] !== score[a]) return score[b] - score[a];
+    if (a < b) return -1;
+    if (a > b) return 1;
+    return 0;
+  });
+
+  return keys.slice(0, 5);
 }
 
 module.exports = problem7;