Solutions

public int[] twoSum(int[] nums, int target) {
Map numsMap = new HashMap<>();

for (int i = 0; i < nums.length; i++) {
int complement = target - nums[i];

if (numsMap.containsKey(complement)) {
return new int[]{numsMap.get(complement), i};
}

numsMap.put(nums[i], i);
}
return new int[]{-1, -1};
}
  

public int lengthOfLongestSubstring(String s) {
    Map<Character, Integer> map = new HashMap<>();
    int left = 0, maxLen = 0;

    for (int right = 0; right < s.length(); right++) {
        char ch = s.charAt(right);
        if (map.containsKey(ch)) {
            left = Math.max(left, map.get(ch) + 1);
        }
        map.put(ch, right);
        maxLen = Math.max(maxLen, right - left + 1);
    }
    return maxLen;
}

public int lengthOfLongestSubstring(String s) {
    Map<Character, Integer> map = new HashMap<>();
    int left = 0, maxLen = 0;

    for (int right = 0; right < s.length(); right++) {
        char ch = s.charAt(right);
        if (map.containsKey(ch)) {
            left = Math.max(left, map.get(ch) + 1);
        }
        map.put(ch, right);
        maxLen = Math.max(maxLen, right - left + 1);
    }
    return maxLen;
}

public String longestPalindrome(String s) {
    if (s == null || s.length() < 1) return "";
    int start = 0, maxLen = 1;

    for (int i = 0; i < s.length(); i++) {
        int len1 = expand(s, i, i);
        int len2 = expand(s, i, i + 1);
        int len = Math.max(len1, len2);
        if (len > maxLen) {
            start = i - (len - 1) / 2;
            maxLen = len;
        }
    }
    return s.substring(start, start + maxLen);
}

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