Solve week 4 problems

counting-bits/bky373.java

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+/**
+ * https://leetcode.com/problems/counting-bits/
+ *
+ * time: O(n * log n)
+ * space: O(1)
+ */
+class Solution {
+
+    public int[] countBits(int n) {
+        int[] ans = new int[n + 1];
+        for (int i = 0; i <= n; i++) {
+            int x = i;
+            int cnt = 0;
+            while (x != 0) {
+                cnt++;
+                x &= (x - 1);
+            }
+            ans[i] = cnt;
+        }
+        return ans;
+    }
+}
+

group-anagrams/bky373.java

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+/**
+ * https://leetcode.com/problems/group-anagrams/
+ *
+ * time: O(n * m log m)
+ * space: O(nm)
+ */
+class Solution {
+
+    public List<List<String>> groupAnagrams(String[] strs) {
+        Map<String, List<String>> groups = new HashMap();
+        for (String str : strs) {
+            char[] arr = str.toCharArray();
+            Arrays.sort(arr);
+            groups.computeIfAbsent(new String(arr), k -> new ArrayList<>())
+                  .add(str);
+        }
+        return new ArrayList(groups.values());
+    }
+}

missing-number/bky373.java

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+/**
+ * https://leetcode.com/problems/missing-number/
+ *
+ * time: O(n)
+ * space: O(1)
+ */
+class Solution {
+    public int missingNumber(int[] nums) {
+        int sum = nums.length * (nums.length+1) / 2;
+        for (int num : nums) {
+            sum -= num;
+        }
+        return sum;
+    }
+}

number-of-1-bits/bky373.java

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+/**
+ * https://leetcode.com/problems/number-of-1-bits/
+ *
+ * time: O(log n)
+ * space: O(1)
+ */
+class Solution {
+
+    public int hammingWeight(int n) {
+        int cnt = 0;
+        while (n != 0) {
+            cnt++;
+            n &= (n - 1);
+        }
+        return cnt;
+    }
+}

reverse-bits/bky373.java

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+/**
+ * https://leetcode.com/problems/reverse-bits/
+ *
+ * time: O(1)
+ * space: O(1)
+ */
+public class Solution {
+
+    public int reverseBits(int n) {
+        int ans = 0;
+        for (int i = 0; i < 32; i++) {
+            ans = ans << 1 | n & 1;
+            n >>= 1;
+        }
+        return ans;
+    }
+}