70. Climbing Stairs

70_climbing_stairs/memo.md

@@ -0,0 +1,19 @@
+# 70. Climbing Stairs
+
+https://leetcode.com/problems/climbing-stairs/
+
+## Comments
+
+### step1
+
+*   dp や再帰で解けるだろう
+*   いくつか試すと、直近 2 つだけ持っていればよい
+
+### step2
+
+*   フィボナッチっぽく実装するならこのあたりとか
+*   for のループ回数とか境界条件をちゃんと確認する必要はある
+
+### step3
+
+*   簡単な問題なので省略。ただし LeetCode に色々別解はある。

70_climbing_stairs/step1.cpp

@@ -0,0 +1,20 @@
+class Solution {
+public:
+    int climbStairs(int n) {
+        if (n <= 1) {
+            return n;
+        }
+        int ways;
+        // Starting from ways(0) = 0
+        int two_steps_before = 0;
+        // ways(1) = 1
+        int one_step_before = 1;
+        // Run n times.
+        for (int i = 0; i < n; ++i) {
+            ways = two_steps_before + one_step_before;
+            two_steps_before = one_step_before;
+            one_step_before = ways;
+        }
+        return ways;
+    }
+};

70_climbing_stairs/step2.cpp

@@ -0,0 +1,16 @@
+class Solution {
+public:
+    int climbStairs(int n) {
+        if (n <= 1) {
+            return n;
+        }
+        int first = 1;
+        int second = 1;
+        for (int i = 2; i <= n; ++i) {
+            int third =  first + second;
+            first = second;
+            second = third;
+        }
+        return second;
+    }
+};