diff --git a/src/problem1.js b/src/problem1.js
index 9a14f2c..afae632 100644
--- a/src/problem1.js
+++ b/src/problem1.js
@@ -1,6 +1,31 @@
 function problem1(pobi, crong) {
-  var answer;
-  return answer;
+  if (pobi[1]-pobi[0] != 1 || crong[1]-crong[0] != 1) return -1;
+  if (Math.min(pobi, crong) < 1 || Math.max(pobi, crong) > 400) return -1;
+
+  const max_pobi = Math.max(...sumAndProduct(pobi));
+  const max_crong = Math.max(...sumAndProduct(crong));
+  
+  if (max_pobi == max_crong) return 0;
+  if (max_pobi > max_crong) return 1;
+  
+  else return 2;
+}
+
+function sumAndProduct(arr){
+  const ret = [];
+
+  arr.forEach(page => {
+    const digits = [];
+
+    while(page > 0){
+      digits.push(page % 10);
+      page = parseInt(page / 10);
+    }
+    ret.push(digits.reduce((acc, cur) => acc + cur, 0)); // 누적 합
+    ret.push(digits.reduce((acc, cur) => acc * cur, 1)); // 누적 곱
+  });
+  
+  return ret;
 }
 
 module.exports = problem1;
diff --git a/src/problem2.js b/src/problem2.js
index cebd07c..9cb1460 100644
--- a/src/problem2.js
+++ b/src/problem2.js
@@ -1,6 +1,12 @@
 function problem2(cryptogram) {
-  var answer;
-  return answer;
+  const stack = [];
+
+  [...cryptogram].forEach(char => {
+    if(stack.at(-1) != char) stack.push(char);
+    else stack.pop();
+  });
+  
+  return stack.toString().replaceAll(',','');
 }
 
 module.exports = problem2;
diff --git a/src/problem3.js b/src/problem3.js
index 1baed28..a703c5a 100644
--- a/src/problem3.js
+++ b/src/problem3.js
@@ -1,6 +1,16 @@
 function problem3(number) {
-  var answer;
-  return answer;
+  let count = 0;
+  for (let i=1; i<=number; ++i) count += count369(i);
+  return count;
+}
+
+function count369(number){
+  let count = 0;
+  while (number > 0){
+    switch (number % 10) {case 3: case 6: case 9: ++count};
+    number = parseInt(number / 10);
+  }
+  return count;
 }
 
 module.exports = problem3;
diff --git a/src/problem4.js b/src/problem4.js
index ee1d3bd..adeda5e 100644
--- a/src/problem4.js
+++ b/src/problem4.js
@@ -1,5 +1,15 @@
 function problem4(word) {
-  var answer;
+  var answer = "";
+
+  [...word].forEach(char => {
+    if (/[A-Z]/.test(char))
+      answer += String.fromCharCode(65+25-(char.charCodeAt()-65));
+    else if (/[a-z]/.test(char))
+      answer += String.fromCharCode(97+25-(char.charCodeAt()-97));
+    else
+      answer += char;
+  });
+  
   return answer;
 }
 
diff --git a/src/problem5.js b/src/problem5.js
index 9368e87..f2f7a9b 100644
--- a/src/problem5.js
+++ b/src/problem5.js
@@ -1,5 +1,10 @@
 function problem5(money) {
-  var answer;
+  const answer = [];
+  const denoms = [50000, 10000, 5000, 1000, 500, 100, 50, 10, 1];
+  denoms.forEach(denom => {
+    answer.push(parseInt(money / denom));
+    money %= denom;
+  });
   return answer;
 }
 
diff --git a/src/problem6.js b/src/problem6.js
index 3f842b5..e83d254 100644
--- a/src/problem6.js
+++ b/src/problem6.js
@@ -1,6 +1,21 @@
 function problem6(forms) {
-  var answer;
-  return answer;
+  const set = new Set();
+  const map = new Map();
+
+  forms.forEach(([email, nickname]) => {
+    for (let i=0; i<nickname.length-1; ++i) {
+      for (let j=i+2; j<=nickname.length; ++j) {
+        const substr = nickname.slice(i, j);
+        if (map.has(substr)) {
+          set.add(email);
+          set.add(map.get(substr));
+        }
+        else map.set(substr, email);
+      }
+    }
+  });
+
+  return Array.from(set).sort();
 }
 
 module.exports = problem6;
diff --git a/src/problem7.js b/src/problem7.js
index ee1bb9d..70fcaf2 100644
--- a/src/problem7.js
+++ b/src/problem7.js
@@ -1,6 +1,40 @@
 function problem7(user, friends, visitors) {
-  var answer;
-  return answer;
+  const map_friends = new Map();
+  const map_scores = new Map();
+
+  friends.forEach(([a,b]) => {
+    if (!map_friends.has(a)) map_friends.set(a, new Set());
+    if (!map_friends.has(b)) map_friends.set(b, new Set());
+    map_friends.get(a).add(b);
+    map_friends.get(b).add(a);
+  });
+
+  const user_friends = map_friends.get(user) || new Set();
+
+  map_friends.forEach((set_friends, friend) => {
+    if (friend === user || user_friends.has(friend)) return;
+    let count = 0;
+    set_friends.forEach(person => {
+      if (user_friends.has(person)) ++count;
+    });
+
+    if (count > 0)
+      map_scores.set(friend, (map_scores.get(friend) || 0) + count * 10);
+  });
+  
+  visitors.forEach(visitor => {
+    if (visitor !== user && !user_friends.has(visitor)){
+      map_scores.set(visitor, (map_scores.get(visitor) || 0) + 1);
+    }
+  });
+
+  const ret = Array.from(map_scores.entries())
+    .filter(([, score]) => score > 0)
+    .sort((a,b) => b[1]-a[1] || a[0].localeCompare(b[0]))
+    .slice(0,5)
+    .map(([friend]) => friend);
+
+  return ret;
 }
 
 module.exports = problem7;