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DecodeWays.swift
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DecodeWays.swift
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//
// DecodeWays.swift
// LeetCode
//
// Created by 榛庤档澶儙 on 08/01/2018.
// Copyright 漏 2018 lizhaotailang. All rights reserved.
//
// A message containing letters from A-Z is being encoded to numbers using the following mapping:
//
// 'A' -> 1
// 'B' -> 2
// ...
// 'Z' -> 26
// Given an encoded message containing digits, determine the total number of ways to decode it.
//
// For example,
// Given encoded message "12", it could be decoded as "AB" (1 2) or "L" (12).
//
// The number of ways decoding "12" is 2.
//
// Accepted. See [DecodeWaysTests](./LeetCodeTests/DecodeWaysTests.swift) for test cases.
//
import Foundation
class DecodeWays {
func numDecodings(_ s: String) -> Int {
if s.isEmpty {
return 0
}
let chars = Array(s)
if chars[0] == "0" {
return 0
}
var ints = Array<Int>.init(repeating: 0, count: s.count + 1)
ints[0] = 1
var i = 1
while i < ints.count {
if chars[i - 1] == "0" {
ints[i] = 0
} else {
ints[i] = ints[i - 1]
}
if i > 1 && (chars[i - 2] == "1" || (chars[i - 2] == "2" && chars[i - 1].unicodeScalars.first!.value <= "6".unicodeScalars.first!.value)) {
ints[i] += ints[i - 2]
}
i += 1
}
return ints[s.count]
}
}