Merge pull request neetcode-gh#166 from r1cky0/patch-12

Create 70-Climbing-Stairs.java

Author: neetcode-gh

java/70-Climbing-Stairs.java

@@ -0,0 +1,51 @@
+//optimal
+class Solution {
+    public int climbStairs(int n) {
+        int a = 1;
+        int b = 1;
+        int c;
+        
+        for (int i = 0; i < n - 1; i++) {
+            c = a + b;
+            a = b;
+            b = c;
+        }
+        return b;
+    }
+}
+
+//bottom up
+class Solution {
+    public int climbStairs(int n) {
+        int[] dp = new int[n + 1];
+        dp[0] = 1;
+        dp[1] = 1;
+        
+        for (int i = 2; i < n + 1; i++) {
+            dp[i] = dp[i - 1] + dp[i - 2];
+        }
+        return dp[n];
+    }
+}
+
+//top down with memo[]
+class Solution {
+    public int climbStairs(int n) {
+        int[] memo = new int[n + 1];
+        Arrays.fill(memo, -1);
+        
+        return climbStairs(n - 1, memo) + climbStairs(n - 2, memo);
+    }
+    
+    private int climbStairs(int n, int[] memo) {
+        if (n < 0) return 0;
+        if (n == 0 || n == 1) {
+            memo[n] = 1;
+            return memo[n];
+        }
+        if (memo[n] != -1) return memo[n];
+        
+        memo[n] = climbStairs(n - 1, memo) + climbStairs(n - 2, memo);
+        return memo[n];
+    }
+}