Merge pull request #1242 from sm9171/ver4/week2

[sm9171] WEEK 02 solutions

Author: sm9171

climbing-stairs/sm9171.java

@@ -0,0 +1,16 @@
+class Solution {
+    public int climbStairs(int n) {
+        int[] memo = new int[n+1];
+        return recur(n, memo);
+    }
+
+    public static int recur(int n, int[] memo) {
+        if (n < 0) return 0;
+        if (n == 0) return 1;
+
+        if (memo[n] > 0) return memo[n];
+
+        memo[n] = recur(n - 1, memo) + recur(n - 2, memo);
+        return memo[n];
+    }
+}

product-of-array-except-self/sm9171.java

@@ -0,0 +1,18 @@
+class Solution {
+    public int[] productExceptSelf(int[] nums) {
+        int[] res = new int[nums.length];
+
+        res[0] = 1;
+        for (int i = 1; i < nums.length; i++) {
+            res[i] = res[i - 1] * nums[i - 1];
+        }
+
+        int acc = 1;
+        for (int i = nums.length - 2; i >= 0; i--) {
+            acc *= nums[i + 1];
+            res[i] *= acc;
+        }
+
+        return res;
+    }
+}

valid-anagram/sm9171.java

@@ -0,0 +1,24 @@
+class Solution {
+    public boolean isAnagram(String s, String t) {
+        if (s.length() != t.length()) {
+            return false;
+        }
+
+        HashMap<Character, Integer> map = new HashMap<>();
+        char[] originString = s.toCharArray();
+        for (int i = 0; i < originString.length; i++) {
+            Integer count = map.getOrDefault(originString[i], 0);
+            map.put(originString[i], count + 1);
+        }
+
+        char[] targetString = t.toCharArray();
+        for (int i = 0; i < targetString.length; i++) {
+            Integer count = map.get(targetString[i]);
+            if (count == null || count == 0) {
+                return false;
+            }
+            map.put(targetString[i], count - 1);
+        }
+        return true;
+    }
+}