Merge pull request neetcode-gh#1242 from aniruddhakj/patch-1

Updating 70.climbing_stairs.cpp

Author: Ahmad-A0

cpp/neetcode_150/13_1-d_dynamic_programming/climbing_stairs.cpp

@@ -2,15 +2,31 @@
     Climbing stairs, either 1 or 2 steps, distinct ways to reach top
     Ex. n = 2 -> 2 (1 + 1, 2), n = 3 -> 3 (1 + 1 + 1, 1 + 2, 2 + 1)
 
-    Recursion w/ memoization -> DP, why DP? Optimal substructure
+    Bottom-up DP
     Recurrence relation: dp[i] = dp[i - 1] + dp[i - 2]
     Reach ith step in 2 ways: 1) 1 step from i-1, 2) 2 steps from i-2
 
     Time: O(n)
     Space: O(1)
 */
 
-class Solution {
+class SolutionBottomUp {
+public:
+    //Fibonacci series : 'one' stores ways from [n-2]
+    //'two' stores ways from [n-1] 
+    int climbStairs(int n) {
+        int one = 1, two = 1;
+        for(int i = 0; i < n - 1; ++i){
+            int temp = one;
+            one += two;
+            two = temp;
+        }
+        return one;
+    }
+};
+
+
+class SolutionTopDown {
 public:
     int climbStairs(int n) {
         if (n == 1) {