[s0ooo0k] WEEK 03 Solutions

combination-sum/s0ooo0k.java

@@ -0,0 +1,23 @@
+class Solution {
+    List<List<Integer>> result = new ArrayList<>();
+    int[] candidates;
+
+    public List<List<Integer>> combinationSum(int[] candidates, int target) {
+        this.candidates=candidates;
+        backtrack(new ArrayList<>(), target, 0);
+        return result;
+    }
+    void backtrack(List<Integer> comb, int remain, int start) {
+        if(remain==0) {
+            result.add(new ArrayList<>(comb));
+            return;
+        }
+        if(remain<0) return;
+        for(int i=start; i<candidates.length; i++) {
+            comb.add(candidates[i]);
+            backtrack(comb, remain-candidates[i], i);
+            comb.remove(comb.size() - 1);
+        }
+    }
+}
+

decode-ways/s0ooo0k.java

@@ -0,0 +1,22 @@
+class Solution {
+    public int numDecodings(String s) {
+        if(s.charAt(0)=='0') return 0;
+
+        int n = s.length();
+        int[] dp = new int[n+1];
+
+        dp[0]=1;
+        dp[1]=1;
+
+        for(int i=2; i<=n; i++) {
+            if(s.charAt(i-1)!='0') dp[i]+=dp[i-1];
+
+            int group = Integer.parseInt(s.substring(i-2, i));
+            if(group>=10 && group<=26) {
+                dp[i]+=dp[i-2];
+            }
+        }
+        return dp[n];
+    }
+}
+

maximum-subarray/s0ooo0k.java

@@ -0,0 +1,13 @@
+class Solution {
+    public int maxSubArray(int[] nums) {
+        int curr = nums[0];
+        int maxSum = nums[0];
+
+        for(int i=1; i<nums.length; i++) {
+            curr = Math.max(nums[i], curr+nums[i]);
+            maxSum = Math.max(maxSum, curr);
+        }
+        return maxSum;
+    }
+}
+

number-of-1-bits/s0ooo0k.java

@@ -0,0 +1,14 @@
+class Solution {
+    public int hammingWeight(int n) {
+        /*
+         * 시간복잡도 O(log n)
+         */
+        int cnt = 0;
+        while(n!=0) {
+            if(n%2==1) cnt++;
+            n = n / 2; 
+        }
+        return cnt;
+    }
+}
+

valid-palindrome/s0ooo0k.java

@@ -0,0 +1,30 @@
+class Solution {
+    /*
+     * 초반 풀이 시 while문 내에서 left<right 미체크로 오류 / 조건 추가
+     * 중복 while문 사용하지 않는 방법이 있는지 고민
+     * 
+     * 시간복잡도 : O(n)
+     */
+    public boolean isPalindrome(String s) {
+        int left = 0;
+        int right = s.length()-1;
+
+        while(left < right) {
+            while(left<right && !Character.isLetterOrDigit(s.charAt(left))) {
+                left++;
+            }
+            while(left<right && !Character.isLetterOrDigit(s.charAt(right))) {
+                right--;
+            }
+
+            if (Character.toLowerCase(s.charAt(left)) != Character.toLowerCase(s.charAt(right))) {
+                return false;
+            }
+
+            left++;
+            right--;
+        }
+        return true;
+    }
+}
+