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

Updating 70.climbing_stairs.cpp

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) {