Solution for Climbing Stairs #230

Add time and space complexity comments to climbStairs method

Author: dohyeon2

climbing-stairs/dohyeon2.java

@@ -0,0 +1,32 @@
+class Solution {
+    // TC: O(n)
+    // SC: O(1)
+    public int climbStairs(int n) {
+        if (n < 3) {
+            // If n is less than 3 there are n ways.
+            return n;
+        }
+
+        // There is one way to reach the first stair.
+        int nMinus2 = 1;
+        // There are two ways to reach the second stair.
+        int nMinus1 = 2;
+
+        int nZero = nMinus1 + nMinus2;
+
+        for (int step = 3; step <= n; step++) {
+            // To reach the third stair, it must come from the first stair or the second
+            // stair.
+            // The number of ways to reach the nth stair is the sum of those of the (n-1)th
+            // stair and those of the (n-2)th stair.
+            nZero = nMinus1 + nMinus2;
+
+            nMinus2 = nMinus1;
+            nMinus1 = nZero;
+        }
+
+        // A separate variable name is used to distinguish between nZero and nMinus1 so
+        // that the nth value is explicit
+        return nZero;
+    }
+}