algo-2/4

소수 찾기.cpp

@@ -0,0 +1,27 @@
+#include <string>
+#include <vector>
+#include <cmath>
+
+using namespace std;
+
+int solution(int n) {
+    int answer = 0;
+    int prime [n+1];
+    for(int i=2; i<=n; i++){
+        prime[i]=i;
+    }
+    for(int i=2; i<=sqrt(n); i++){
+        if(prime[i]==0)
+            continue;
+        // i*k (k<i) 까지의 수는 이미 검사했으므로 j는 i*i 부터 검사해준다.
+        for (int j = i * i; j <= n; j += i)
+        { prime[j] = 0;}
+    }
+    for (int i = 2; i <= n; i++)
+    {
+        if (prime[i] != 0)
+          answer++;
+    }
+
+    return answer;
+}

시저 암호.cpp

@@ -0,0 +1,32 @@
+#include <string>
+#include <vector>
+#include <iostream>
+
+using namespace std;
+
+string solution(string s, int n) {
+    string answer = "";
+    for(int i=0; i<s.size(); i++){
+    if(s[i]==' '){
+            answer.push_back(' ');
+            continue;
+    }
+    if(s[i]>96){
+        if(s[i]+n>122){
+            answer.push_back(s[i]+n-26);
+        }
+        else{
+            answer.push_back(s[i]+n);
+        }
+    }
+    else {
+        if(s[i]+n>90){
+            answer.push_back(s[i]+n-26);
+        }
+        else{
+            answer.push_back(s[i]+n);
+        }
+    }
+    }
+    return answer;
+}

이상한 문자 만들기.cpp

@@ -0,0 +1,32 @@
+#include <string>
+#include <vector>
+
+using namespace std;
+
+string solution(string s) {
+    string answer = "";
+    int idx =0;
+    for(int i=0; i<s.size(); i++){
+        if(s[i]==' '){
+            answer.push_back(' ');
+            idx =0;
+            continue;
+        }
+        if(idx%2==0){
+            if(s[i] >= 'a' && s[i] <= 'z')
+            { answer.push_back(s[i]- 'a' + 'A');}
+        else
+        {answer.push_back(s[i]);
+        }
+        }
+        else{
+           if(s[i] >= 'A' && s[i] <= 'Z')
+            { answer.push_back(s[i]- 'A' + 'a');}
+        else
+        {answer.push_back(s[i]);
+        }
+        }
+         idx++;
+    }
+    return answer;
+}