diff --git a/codes/swift/chapter_computational_complexity/recursion.swift b/codes/swift/chapter_computational_complexity/recursion.swift index 5b4012cc..9d3fedc9 100644 --- a/codes/swift/chapter_computational_complexity/recursion.swift +++ b/codes/swift/chapter_computational_complexity/recursion.swift @@ -16,6 +16,25 @@ func recur(n: Int) -> Int { return n + res } +/* 使用迭代模拟递归 */ +func forLoopRecur(n: Int) -> Int { + // 使用一个显式的栈来模拟系统调用栈 + var stack: [Int] = [] + var res = 0 + // 递:递归调用 + for i in stride(from: n, to: 0, by: -1) { + // 通过“入栈操作”模拟“递” + stack.append(i) + } + // 归:返回结果 + while !stack.isEmpty { + // 通过“出栈操作”模拟“归” + res += stack.removeLast() + } + // res = 1+2+3+...+n + return res +} + /* 尾递归 */ func tailRecur(n: Int, res: Int) -> Int { // 终止条件 @@ -48,6 +67,9 @@ enum Recursion { res = recursion.recur(n: n) print("\n递归函数的求和结果 res = \(res)") + res = recursion.forLoopRecur(n: n) + print("\n使用迭代模拟递归求和结果 res = \(res)") + res = recursion.tailRecur(n: n, res: 0) print("\n尾递归函数的求和结果 res = \(res)") diff --git a/codes/swift/chapter_dynamic_programming/min_cost_climbing_stairs_dp.swift b/codes/swift/chapter_dynamic_programming/min_cost_climbing_stairs_dp.swift index 21766555..a86a86bb 100644 --- a/codes/swift/chapter_dynamic_programming/min_cost_climbing_stairs_dp.swift +++ b/codes/swift/chapter_dynamic_programming/min_cost_climbing_stairs_dp.swift @@ -13,8 +13,8 @@ func minCostClimbingStairsDP(cost: [Int]) -> Int { // 初始化 dp 表,用于存储子问题的解 var dp = Array(repeating: 0, count: n + 1) // 初始状态:预设最小子问题的解 - dp[1] = 1 - dp[2] = 2 + dp[1] = cost[1] + dp[2] = cost[2] // 状态转移:从较小子问题逐步求解较大子问题 for i in stride(from: 3, through: n, by: 1) { dp[i] = min(dp[i - 1], dp[i - 2]) + cost[i] diff --git a/codes/swift/chapter_hashing/hash_map_open_addressing.swift b/codes/swift/chapter_hashing/hash_map_open_addressing.swift index 53e01da2..61ef7912 100644 --- a/codes/swift/chapter_hashing/hash_map_open_addressing.swift +++ b/codes/swift/chapter_hashing/hash_map_open_addressing.swift @@ -13,7 +13,7 @@ class HashMapOpenAddressing { var loadThres: Double // 触发扩容的负载因子阈值 var extendRatio: Int // 扩容倍数 var buckets: [Pair?] // 桶数组 - var removed: Pair // 删除标记 + var TOMBSTONE: Pair // 删除标记 /* 构造方法 */ init() { @@ -22,7 +22,7 @@ class HashMapOpenAddressing { loadThres = 2.0 / 3.0 extendRatio = 2 buckets = Array(repeating: nil, count: capacity) - removed = Pair(key: -1, val: "-1") + TOMBSTONE = Pair(key: -1, val: "-1") } /* 哈希函数 */ @@ -35,22 +35,42 @@ class HashMapOpenAddressing { Double(size / capacity) } + /* 搜索 key 对应的桶索引 */ + func findBucket(key: Int) -> Int { + var index = hashFunc(key: key) + var firstTombstone = -1 + // 线性探测,当遇到空桶时跳出 + while buckets[index] != nil { + // 若遇到 key ,返回对应桶索引 + if buckets[index]!.key == key { + // 若之前遇到了删除标记,则将键值对移动至该索引 + if firstTombstone != -1 { + buckets[firstTombstone] = buckets[index] + buckets[index] = TOMBSTONE + return firstTombstone // 返回移动后的桶索引 + } + return index // 返回桶索引 + } + // 记录遇到的首个删除标记 + if firstTombstone == -1 && buckets[index] == TOMBSTONE { + firstTombstone = index + } + // 计算桶索引,越过尾部返回头部 + index = (index + 1) % capacity + } + // 若 key 不存在,则返回添加点的索引 + return firstTombstone == -1 ? index : firstTombstone + } + /* 查询操作 */ func get(key: Int) -> String? { - let index = hashFunc(key: key) - // 线性探测,从 index 开始向后遍历 - for i in stride(from: 0, to: capacity, by: 1) { - // 计算桶索引,越过尾部返回头部 - let j = (index + i) % capacity - // 若遇到空桶,说明无此 key ,则返回 nil - if buckets[j] == nil { - return nil - } - // 若遇到指定 key ,则返回对应 val - if buckets[j]?.key == key, buckets[j] != removed { - return buckets[j]?.val - } + // 搜索 key 对应的桶索引 + let index = findBucket(key: key) + // 若找到键值对,则返回对应 val + if buckets[index] != nil, buckets[index] != TOMBSTONE { + return buckets[index]!.val } + // 若键值对不存在,则返回 null return nil } @@ -60,42 +80,26 @@ class HashMapOpenAddressing { if loadFactor() > loadThres { extend() } - let index = hashFunc(key: key) - // 线性探测,从 index 开始向后遍历 - for i in stride(from: 0, through: capacity, by: 1) { - // 计算桶索引,越过尾部返回头部 - let j = (index + i) % capacity - // 若遇到空桶、或带有删除标记的桶,则将键值对放入该桶 - if buckets[j] == nil || buckets[j] == removed { - buckets[j] = Pair(key: key, val: val) - size += 1 - return - } - // 若遇到指定 key ,则更新对应 val - if buckets[j]?.key == key { - buckets[j]?.val = val - return - } + // 搜索 key 对应的桶索引 + let index = findBucket(key: key) + // 若找到键值对,则覆盖 val 并返回 + if buckets[index] != nil, buckets[index] != TOMBSTONE { + buckets[index]!.val = val + return } + // 若键值对不存在,则添加该键值对 + buckets[index] = Pair(key: key, val: val) + size += 1 } /* 删除操作 */ func remove(key: Int) { - let index = hashFunc(key: key) - // 线性探测,从 index 开始向后遍历 - for i in stride(from: 0, to: capacity, by: 1) { - // 计算桶索引,越过尾部返回头部 - let j = (index + i) % capacity - // 若遇到空桶,说明无此 key ,则直接返回 - if buckets[j] == nil { - return - } - // 若遇到指定 key ,则标记删除并返回 - if buckets[j]?.key == key { - buckets[j] = removed - size -= 1 - return - } + // 搜索 key 对应的桶索引 + let index = findBucket(key: key) + // 若找到键值对,则用删除标记覆盖它 + if buckets[index] != nil, buckets[index] != TOMBSTONE { + buckets[index] = TOMBSTONE + size -= 1 } } @@ -109,7 +113,7 @@ class HashMapOpenAddressing { size = 0 // 将键值对从原哈希表搬运至新哈希表 for pair in bucketsTmp { - if let pair, pair != removed { + if let pair, pair != TOMBSTONE { put(key: pair.key, val: pair.val) } } @@ -118,10 +122,12 @@ class HashMapOpenAddressing { /* 打印哈希表 */ func print() { for pair in buckets { - if let pair { - Swift.print("\(pair.key) -> \(pair.val)") - } else { + if pair == nil { Swift.print("null") + } else if pair == TOMBSTONE { + Swift.print("TOMBSTONE") + } else { + Swift.print("\(pair!.key) -> \(pair!.val)") } } }