#771 · #3223 · Minimum Length of String After Operations(操作后字符串的最短长度)

Given a string s, an operation picks an index i and a character c equal to s[i] that appears both to the left and to the right of i, then deletes the closest such occurrence on each side. Return the minimum length of s after any number of operations. Constraints: 1 <= s.length <= 2 * 10^5, s consists of lowercase English letters.