Added tasks 3692-3700

src/main/java/g3601_3700/s3692_majority_frequency_characters/Solution.java

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+package g3601_3700.s3692_majority_frequency_characters;
+
+// #Easy #Biweekly_Contest_166 #2025_09_28_Time_1_ms_(100.00%)_Space_43.06_MB_(100.00%)
+
+public class Solution {
+    public String majorityFrequencyGroup(String s) {
+        int[] cntArray = new int[26];
+        for (int i = 0; i < s.length(); i++) {
+            cntArray[s.charAt(i) - 'a']++;
+        }
+        int[] freq = new int[s.length() + 1];
+        for (int i = 0; i < 26; i++) {
+            if (cntArray[i] > 0) {
+                freq[cntArray[i]]++;
+            }
+        }
+        int size = 0;
+        int bfreq = 0;
+        for (int i = 0; i <= s.length(); i++) {
+            int si = freq[i];
+            if (si > size || (si == size && i > bfreq)) {
+                size = si;
+                bfreq = i;
+            }
+        }
+        StringBuilder sb = new StringBuilder();
+        for (int i = 0; i < 26; i++) {
+            if (cntArray[i] == bfreq) {
+                sb.append((char) (i + 'a'));
+            }
+        }
+        return sb.toString();
+    }
+}

src/main/java/g3601_3700/s3692_majority_frequency_characters/readme.md

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+3692\. Majority Frequency Characters
+
+Easy
+
+You are given a string `s` consisting of lowercase English letters.
+
+The **frequency group** for a value `k` is the set of characters that appear exactly `k` times in s.
+
+The **majority frequency group** is the frequency group that contains the largest number of **distinct** characters.
+
+Return a string containing all characters in the majority frequency group, in **any** order. If two or more frequency groups tie for that largest size, pick the group whose frequency `k` is **larger**.
+
+**Example 1:**
+
+**Input:** s = "aaabbbccdddde"
+
+**Output:** "ab"
+
+**Explanation:**
+
+| Frequency (k) | Distinct characters in group | Group size | Majority? |
+|---------------|------------------------------|------------|-----------|
+| 4             | {d}                          | 1          | No        |
+| 3             | {a, b}                       | 2          | **Yes**   |
+| 2             | {c}                          | 1          | No        |
+| 1             | {e}                          | 1          | No        |
+
+Both characters `'a'` and `'b'` share the same frequency 3, they are in the majority frequency group. `"ba"` is also a valid answer.
+
+**Example 2:**
+
+**Input:** s = "abcd"
+
+**Output:** "abcd"
+
+**Explanation:**
+
+| Frequency (k) | Distinct characters in group | Group size | Majority? |
+|---------------|------------------------------|------------|-----------|
+| 1             | {a, b, c, d}                 | 4          | **Yes**   |
+
+All characters share the same frequency 1, they are all in the majority frequency group.
+
+**Example 3:**
+
+**Input:** s = "pfpfgi"
+
+**Output:** "fp"
+
+**Explanation:**
+
+| Frequency (k) | Distinct characters in group | Group size | Majority?                        |
+|---------------|------------------------------|------------|----------------------------------|
+| 2             | {p, f}                       | 2          | **Yes**                          |
+| 1             | {g, i}                       | 2          | No (tied size, lower frequency)  |
+
+Both characters `'p'` and `'f'` share the same frequency 2, they are in the majority frequency group. There is a tie in group size with frequency 1, but we pick the higher frequency: 2.
+
+**Constraints:**
+
+*   `1 <= s.length <= 100`
+*   `s` consists only of lowercase English letters.

src/main/java/g3601_3700/s3693_climbing_stairs_ii/Solution.java

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+package g3601_3700.s3693_climbing_stairs_ii;
+
+// #Medium #Biweekly_Contest_166 #2025_09_28_Time_2_ms_(100.00%)_Space_57.06_MB_(100.00%)
+
+@SuppressWarnings({"unused", "java:S1172"})
+public class Solution {
+    public int climbStairs(int n, int[] costs) {
+        if (costs.length == 1) {
+            return costs[0] + 1;
+        }
+        int one = costs[0] + 1;
+        int two = Math.min(one + costs[1] + 1, costs[1] + 4);
+        if (costs.length < 3) {
+            return two;
+        }
+        int three = Math.min(one + costs[2] + 4, Math.min(two + costs[2] + 1, costs[2] + 9));
+        if (costs.length < 4) {
+            return three;
+        }
+        for (int i = 3; i < costs.length; i++) {
+            int four =
+                    (Math.min(
+                            three + costs[i] + 1,
+                            Math.min(two + costs[i] + 4, one + costs[i] + 9)));
+            one = two;
+            two = three;
+            three = four;
+        }
+        return three;
+    }
+}

src/main/java/g3601_3700/s3693_climbing_stairs_ii/readme.md

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+3693\. Climbing Stairs II
+
+Medium
+
+You are climbing a staircase with `n + 1` steps, numbered from 0 to `n`.
+
+You are also given a **1-indexed** integer array `costs` of length `n`, where `costs[i]` is the cost of step `i`.
+
+From step `i`, you can jump **only** to step `i + 1`, `i + 2`, or `i + 3`. The cost of jumping from step `i` to step `j` is defined as: <code>costs[j] + (j - i)<sup>2</sup></code>
+
+You start from step 0 with `cost = 0`.
+
+Return the **minimum** total cost to reach step `n`.
+
+**Example 1:**
+
+**Input:** n = 4, costs = [1,2,3,4]
+
+**Output:** 13
+
+**Explanation:**
+
+One optimal path is `0 → 1 → 2 → 4`
+
+Jump
+
+Cost Calculation
+
+Cost
+
+0 → 1
+
+<code>costs[1] + (1 - 0)<sup>2</sup> = 1 + 1</code>
+
+2
+
+1 → 2
+
+<code>costs[2] + (2 - 1)<sup>2</sup> = 2 + 1</code>
+
+3
+
+2 → 4
+
+<code>costs[4] + (4 - 2)<sup>2</sup> = 4 + 4</code>
+
+8
+
+Thus, the minimum total cost is `2 + 3 + 8 = 13`
+
+**Example 2:**
+
+**Input:** n = 4, costs = [5,1,6,2]
+
+**Output:** 11
+
+**Explanation:**
+
+One optimal path is `0 → 2 → 4`
+
+Jump
+
+Cost Calculation
+
+Cost
+
+0 → 2
+
+<code>costs[2] + (2 - 0)<sup>2</sup> = 1 + 4</code>
+
+5
+
+2 → 4
+
+<code>costs[4] + (4 - 2)<sup>2</sup> = 2 + 4</code>
+
+6
+
+Thus, the minimum total cost is `5 + 6 = 11`
+
+**Example 3:**
+
+**Input:** n = 3, costs = [9,8,3]
+
+**Output:** 12
+
+**Explanation:**
+
+The optimal path is `0 → 3` with total cost = <code>costs[3] + (3 - 0)<sup>2</sup> = 3 + 9 = 12</code>
+
+**Constraints:**
+
+*   <code>1 <= n == costs.length <= 10<sup>5</sup></code>
+*   <code>1 <= costs[i] <= 10<sup>4</sup></code>

src/main/java/g3601_3700/s3694_distinct_points_reachable_after_substring_removal/Solution.java

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+package g3601_3700.s3694_distinct_points_reachable_after_substring_removal;
+
+// #Medium #Biweekly_Contest_166 #2025_09_28_Time_37_ms_(100.00%)_Space_46.42_MB_(100.00%)
+
+import java.util.HashSet;
+import java.util.Set;
+
+public class Solution {
+    public int distinctPoints(String s, int k) {
+        Set<Long> seen = new HashSet<>();
+        seen.add(0L);
+        int x = 0;
+        int y = 0;
+        for (int i = k; i < s.length(); i++) {
+            // add new step
+            switch (s.charAt(i)) {
+                case 'U':
+                    y++;
+                    break;
+                case 'D':
+                    y--;
+                    break;
+                case 'L':
+                    x++;
+                    break;
+                case 'R':
+                default:
+                    x--;
+                    break;
+            }
+            // remove old step
+            switch (s.charAt(i - k)) {
+                case 'U':
+                    y--;
+                    break;
+                case 'D':
+                    y++;
+                    break;
+                case 'L':
+                    x--;
+                    break;
+                case 'R':
+                default:
+                    x++;
+                    break;
+            }
+            seen.add(1000000L * x + y);
+        }
+        return seen.size();
+    }
+}

src/main/java/g3601_3700/s3694_distinct_points_reachable_after_substring_removal/readme.md

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+3694\. Distinct Points Reachable After Substring Removal
+
+Medium
+
+You are given a string `s` consisting of characters `'U'`, `'D'`, `'L'`, and `'R'`, representing moves on an infinite 2D Cartesian grid.
+
+*   `'U'`: Move from `(x, y)` to `(x, y + 1)`.
+*   `'D'`: Move from `(x, y)` to `(x, y - 1)`.
+*   `'L'`: Move from `(x, y)` to `(x - 1, y)`.
+*   `'R'`: Move from `(x, y)` to `(x + 1, y)`.
+
+You are also given a positive integer `k`.
+
+You **must** choose and remove **exactly one** contiguous substring of length `k` from `s`. Then, start from coordinate `(0, 0)` and perform the remaining moves in order.
+
+Return an integer denoting the number of **distinct** final coordinates reachable.
+
+**Example 1:**
+
+**Input:** s = "LUL", k = 1
+
+**Output:** 2
+
+**Explanation:**
+
+After removing a substring of length 1, `s` can be `"UL"`, `"LL"` or `"LU"`. Following these moves, the final coordinates will be `(-1, 1)`, `(-2, 0)` and `(-1, 1)` respectively. There are two distinct points `(-1, 1)` and `(-2, 0)` so the answer is 2.
+
+**Example 2:**
+
+**Input:** s = "UDLR", k = 4
+
+**Output:** 1
+
+**Explanation:**
+
+After removing a substring of length 4, `s` can only be the empty string. The final coordinates will be `(0, 0)`. There is only one distinct point `(0, 0)` so the answer is 1.
+
+**Example 3:**
+
+**Input:** s = "UU", k = 1
+
+**Output:** 1
+
+**Explanation:**
+
+After removing a substring of length 1, `s` becomes `"U"`, which always ends at `(0, 1)`, so there is only one distinct final coordinate.
+
+**Constraints:**
+
+*   <code>1 <= s.length <= 10<sup>5</sup></code>
+*   `s` consists of only `'U'`, `'D'`, `'L'`, and `'R'`.
+*   `1 <= k <= s.length`

src/main/java/g3601_3700/s3695_maximize_alternating_sum_using_swaps/Solution.java

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+package g3601_3700.s3695_maximize_alternating_sum_using_swaps;
+
+// #Hard #Biweekly_Contest_166 #2025_09_28_Time_27_ms_(99.82%)_Space_106.96_MB_(100.00%)
+
+import java.util.ArrayList;
+import java.util.List;
+
+@SuppressWarnings("unchecked")
+public class Solution {
+    private int[] root;
+
+    public long maxAlternatingSum(int[] nums, int[][] swaps) {
+        int n = nums.length;
+        root = new int[n];
+        List<Integer>[] list = new ArrayList[n];
+        int[] oddCount = new int[n];
+        for (int i = 0; i < n; i++) {
+            root[i] = i;
+            list[i] = new ArrayList<>();
+        }
+        for (int[] s : swaps) {
+            union(s[0], s[1]);
+        }
+        for (int i = 0; i < n; i++) {
+            int r = findRoot(i);
+            list[r].add(nums[i]);
+            if (i % 2 == 1) {
+                oddCount[r]++;
+            }
+        }
+        long result = 0;
+        for (int i = 0; i < n; i++) {
+            int r = root[i];
+            if (r != i) {
+                continue;
+            }
+            list[i].sort((a, b) -> a - b);
+            for (int j = 0; j < list[i].size(); j++) {
+                result += (long) list[i].get(j) * (j < oddCount[r] ? -1 : 1);
+            }
+        }
+        return result;
+    }
+
+    private void union(int a, int b) {
+        a = findRoot(a);
+        b = findRoot(b);
+        if (a == b) {
+            return;
+        }
+        if (a < b) {
+            root[b] = a;
+        } else {
+            root[a] = b;
+        }
+    }
+
+    private int findRoot(int a) {
+        if (a == root[a]) {
+            return a;
+        }
+        root[a] = findRoot(root[a]);
+        return root[a];
+    }
+}