solve climbing stairs

Author: sora0319

climbing-stairs/sora0319.java

@@ -1,3 +1,25 @@
+// 공간 복잡도를 줄인 2번째 버전
+class Solution {
+    public int climbStairs(int n) {
+        int back = 1;
+        int front = 2;
+        int target = 0;
+
+        if (n == 1) return back;
+        if (n == 2) return front;
+        
+        for(int i = 1; i < n-1; i++){
+            target = front + back;
+            back = front;
+            front = target;
+        }
+
+        return target;
+    }
+}
+
+
+// 초기 버전
 class Solution {
     public int climbStairs(int n) {
         int[] stairs = new int[n+1];
@@ -12,3 +34,8 @@ public int climbStairs(int n) {
     }
 }
 
+// 1 : 1
+// 2 : [1] + 1, 2                           1+1 2  2개
+// 3 : [2-1] + 1, [2-2] + 1, [1] + 2        1+1+1  2+1  1+2  3개
+// 4 : [3-1] + 1, [3-2] + 1, [3-3] + 1      1+1+1+1  2+1+1   1+2+1  1+1+2  2+2  5개
+// 5 :                                      1+1+1+1+1 2+1+1+1  1+2+1+1  1+1+2+1  1+1+1+2  2+2+1  2+1+2  1+2+2  8개