[jinhyungrhee] WEEK 02 solutions

climbing-stairs/jinhyungrhee.java

@@ -0,0 +1,67 @@
+import java.util.*;
+class Solution {
+    public int climbStairs(int n) {
+
+        // METHOD1 : recursive DFS
+        // int resuslt = recursiveDFS(0, n);
+
+        // METHOD2 : recursive DFS + memoization
+        // int[] memo = new int[n + 1];
+        // Arrays.fill(memo, -1);
+        // int result = memoizationDFS(0, n, memo);
+
+        // METHOD3 : DP
+        int[] memo = new int[n + 1];
+        Arrays.fill(memo, -1);
+        int result = dp(n, memo);
+
+        return result;
+    }
+
+    /**
+     runtime : 0ms
+     memory : 40.04mb
+     */
+
+    // METHOD3 : DP (Bottom-Up)
+    // time-complexity : O(N)
+    // space-complexity : O(N)
+    public int dp(int n, int[] memo) {
+        if (n <= 2) return n;
+        memo[1] = 1;
+        memo[2] = 2;
+        for (int i = 3; i < n + 1; i++) {
+            memo[i] = memo[i - 1] + memo[i - 2];
+        }
+        return memo[n];
+    }
+
+    /**
+     runtime : 0ms
+     memory : 40.30mb
+     */
+
+    // METHOD2 : DFS + memoization (Top-Down)
+    // time-complexity : O(N) -> 각 i에 대해 dfs(i)는 최대 한번만 호출됨
+    // space-complexity : O(N)
+    public int memoizationDFS(int i, int n, int[] memo) {
+        if (i > n) return 0;
+        if (i == n) return 1;
+        if (memo[i] != -1) return memo[i];
+        memo[i] = memoizationDFS(i + 1 , n, memo) + memoizationDFS(i + 2, n, memo);
+        return memo[i];
+    }
+
+    /**
+     Time Limit Exceeded
+     */
+
+    // METHOD1 : recursive DFS => TIME-OUT 발생
+    // time-complexity : O(2^N) -> n이 커질수록 중복 호출 발생
+    // space-complexity : O(N)
+    public int recursiveDFS(int i, int n) {
+        if (i > n) return 0;
+        if (i == n) return 1;
+        return recursiveDFS(i + 1, n) + recursiveDFS(i + 2, n);
+    }
+}