Solve climbing-stairs

Author: OstenHun

climbing-stairs/OstenHun.cpp

@@ -0,0 +1,70 @@
+/*
+    You are climbing a staircase. It takes n steps to reach the top.
+
+    Each time you can either climb 1 or 2 steps. 
+    In how many distinct ways can you climb to the top?
+
+    Example 1:
+
+    Input: n = 2
+    Output: 2
+    Explanation: There are two ways to climb to the top.
+    1. 1 step + 1 step
+    2. 2 steps
+
+    Constraints:
+    1 <= n <= 45
+*/
+
+// TimeComplexity : O(n)
+// SpaceComplexity : O(n)
+#include <iostream>
+using namespace std;
+
+#pragma region DpArrayIdea
+namespace dp_array_idea {
+
+class Solution {
+public:
+    int climbStairs(int n) {
+        int dp[45];
+
+        // dp[i]는 i-1번째 칸을 오르는 경우의 수를 말한다.
+        dp[0] = 1;
+        dp[1] = 2;
+
+        for (int i = 2; i < n; i++) {
+            dp[i] = dp[i-1] + dp[i-2];
+        }
+
+        return dp[n-1];
+    }
+};
+
+}  // namespace dp_array_idea
+#pragma endregion
+
+// 배열을 사용하지 않는 풀이
+// Time : O(n)
+// Space : O(1)
+#pragma region FinalSolution
+class Solution {
+public:
+    int climbStairs(int n) {
+        if (n <= 2) {
+            return n;
+        }
+
+        int one_back = 1;
+        int two_back = 2;
+
+        for (int i = 3; i <= n; i++) {
+            int current = one_back + two_back;
+            two_back = one_back;
+            one_back = current;
+        }
+
+        return one_back;
+    }
+};
+#pragma endregion