Array and Strings: Medium

Two sum

class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        Arrays.sort(nums);
        List<List<Integer>> res = new ArrayList<>();
        for (int i = 0; i < nums.length && nums[i] <= 0; ++i)
            if (i == 0 || nums[i - 1] != nums[i]) {
                twoSum(nums, i, res);
            }
        return res;
    }
    void twoSum(int[] nums, int i, List<List<Integer>> res) {
        var seen = new HashSet<Integer>();
        for (int j = i + 1; j < nums.length; ++j) {
            int complement = -nums[i] - nums[j];
            if (seen.contains(complement)) {
                res.add(Arrays.asList(nums[i], nums[j], complement));
                while (j + 1 < nums.length && nums[j] == nums[j + 1])
                    ++j;
            }
            seen.add(nums[j]);
        }
    }
}

Set Matrix to Zeros

class Solution {
  public void setZeroes(int[][] matrix) {
    int R = matrix.length;
    int C = matrix[0].length;
    Set<Integer> rows = new HashSet<Integer>();
    Set<Integer> cols = new HashSet<Integer>();

    for (int i = 0; i < R; i++) {
      for (int j = 0; j < C; j++) {
        if (matrix[i][j] == 0) {
          rows.add(i);
          cols.add(j);
        }
      }
    }
      
    for (int i = 0; i < R; i++) {
      for (int j = 0; j < C; j++) {
        if (rows.contains(i) || cols.contains(j)) {
          matrix[i][j] = 0;
        }
      }
    }
  }
}

Group Anagrams

class Solution {
    public List<List<String>> groupAnagrams(String[] strs) {
        if (strs == null || strs.length == 0) return new ArrayList<>();
        Map<String, List<String>> map = new HashMap<>();
        for (String s : strs) {
            char[] ca = new char[26];
            for (char c : s.toCharArray()) ca[c - 'a']++;
            String keyStr = String.valueOf(ca);
            if (!map.containsKey(keyStr)) map.put(keyStr, new ArrayList<>());
            map.get(keyStr).add(s);
        }
        return new ArrayList<>(map.values());
    
    }
}

Longest Substring Without Repeating Characters

class Solution {
    public int lengthOfLongestSubstring(String s) {
        Set<Character> ch = new HashSet<Character>();
        
        int result = 0;
        int st = 0, en = 0;
        
        
        while (st < s.length() && en < s.length()) {
            if (!ch.contains(s.charAt(en))) {
                ch.add(s.charAt(en));
                en++;
                result = Math.max(result, en - st);
            } else {
                ch.remove(s.charAt(st));
                st++;
            }
        }
        
        return result;
       
    }
}

Longest Palindromic Substring

class Solution {
    public String longestPalindrome(String s) {
    if (s == null || s.length() < 1) return "";
    int start = 0, end = 0;
    for (int i = 0; i < s.length(); i++) {
        int len1 = expandAroundCenter(s, i, i);
        int len2 = expandAroundCenter(s, i, i + 1);
        int len = Math.max(len1, len2);
        if (len > end - start) {
            start = i - (len - 1) / 2;
            end = i + len / 2;
        }
    }
    return s.substring(start, end + 1);
}

private int expandAroundCenter(String s, int left, int right) {
    int L = left, R = right;
    while (L >= 0 && R < s.length() && s.charAt(L) == s.charAt(R)) {
        L--;
        R++;
    }
    return R - L - 1;
}
}

Increasing Triplet Subsequence

class Solution {
    public boolean increasingTriplet(int[] nums) {
        
        int first = Integer.MAX_VALUE;
        int sec = Integer.MAX_VALUE;
        
        for (int n : nums) {
            if (n <= first) {
                first = n;
            } else if (n <= sec) {
                sec = n;
            } else {
                return true; 
            }
        }
        return false;
        
    }
}