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  Given a positive integer   n , find the   pivot integer   x   such that: The sum of all elements between  1  and  x  inclusively equals the sum of all elements between  x  and  n  inclusively. Return  the pivot integer  x . If no such integer exists, return  -1 . It is guaranteed that there will be at most one pivot index for the given input.   Example 1: Input: n = 8 Output: 6 Explanation: 6 is the pivot integer since: 1 + 2 + 3 + 4 + 5 + 6 = 6 + 7 + 8 = 21. Example 2: Input: n = 1 Output: 1 Explanation: 1 is the pivot integer since: 1 = 1. Example 3: Input: n = 4 Output: -1 Explanation: It can be proved that no such integer exist.   Constraints: 1 <= n <= 1000 Solution : class Solution { publ ic:     int pivotInteger( int n ) {         int sum = (( n )*( n + 1 ))/ 2 ;         int i = 1 ;         int j = n ;         while(i <= j )         {             int mid = (i +j)/ 2 ;             int sum1 = ( mid *( mid + 1 ))/ 2 ;             if (sum1 == sum -

Regular Expression Matching Given an input string s and a pattern p, implement regular expression matching with support for '.' and '*' where: '.' Matches any single character.​​​​ '*' Matches zero or more of the preceding element. The matching should cover the entire input string (not partial). Example 1: Input: s = "aa", p = "a"  Output: false  Explanation: "a" does not match the entire string "aa". Example 2: Input: s = "aa", p = "a*"  Output: true  Explanation: '*' means zero or more of the preceding element, 'a'. Therefore, by repeating 'a' once, it becomes "aa". Example 3: Input: s = "ab", p = ".*"  Output: true  Explanation: ".*" means "zero or more (*) of any character (.)". Constraints: 1 <= s.length <= 20 1 <= p.length <= 20 s contains only lowercase English letters. p contains only lowercase Englis