I tried to use swift to solve an algorithm problem, but it gave me an error Problem Link: https://leetcode.com/problems/number-of-matching-subsequences/ class Solution { func bisect_right(arr: [Int], num: Int) -> Int { var l = 0, r = arr.count - 1 while l < r { var m = (l+r) / 2 if arr[m] <= num { l = m + 1 } else { r = m } } return l } func numMatchingSubseq(_ s: String, _ words: [String]) -> Int { var pos: [Character: [Int]] = [:] for (i, c) in s.enumerated() { pos[c]!.append(i) } var cnt = 0 for word in words { var prev = -1, ok = true for c in word { let ps = pos[c]! var res = bisect_right(arr: ps, num: prev) if res == ps.count { ok = false; break } prev = ps[res] } if ok { cnt += 1 } } return cnt } } i.e. s= "abcde" words = ["a","bb","acd","ace"]