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55 | 55 |
|
56 | 56 | <!-- 这里可写通用的实现逻辑 -->
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57 | 57 |
|
| 58 | +**方法一:单调栈 + 动态规划** |
| 59 | + |
| 60 | +根据题目描述,我们实际上需要找到 $nums[i]$ 的下一个大于等于 $nums[i]$ 的位置 $j$,以及下一个小于 $nums[i]$ 的位置 $j$。我们利用单调栈可以在 $O(n)$ 的时间内找到这两个位置,然后构建邻接表 $g$,其中 $g[i]$ 表示下标 $i$ 可以跳转到的下标。 |
| 61 | + |
| 62 | +然后我们使用动态规划求解最小代价。设 $f[i]$ 表示跳转到下标 $i$ 的最小代价,初始时 $f[0] = 0$,其余 $f[i] = \infty$。我们从小到大枚举下标 $i$,对于每个 $i$,我们枚举 $g[i]$ 中的每个下标 $j$,进行状态转移 $f[j] = \min(f[j], f[i] + costs[j])$。答案为 $f[n - 1]$。 |
| 63 | + |
| 64 | +时间复杂度 $O(n)$,空间复杂度 $O(n)$。其中 $n$ 为数组长度。 |
| 65 | + |
58 | 66 | <!-- tabs:start -->
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59 | 67 |
|
60 | 68 | ### **Python3**
|
61 | 69 |
|
62 | 70 | <!-- 这里可写当前语言的特殊实现逻辑 -->
|
63 | 71 |
|
64 | 72 | ```python
|
65 |
| - |
| 73 | +class Solution: |
| 74 | + def minCost(self, nums: List[int], costs: List[int]) -> int: |
| 75 | + n = len(nums) |
| 76 | + g = defaultdict(list) |
| 77 | + stk = [] |
| 78 | + for i in range(n - 1, -1, -1): |
| 79 | + while stk and nums[stk[-1]] < nums[i]: |
| 80 | + stk.pop() |
| 81 | + if stk: |
| 82 | + g[i].append(stk[-1]) |
| 83 | + stk.append(i) |
| 84 | + |
| 85 | + stk = [] |
| 86 | + for i in range(n - 1, -1, -1): |
| 87 | + while stk and nums[stk[-1]] >= nums[i]: |
| 88 | + stk.pop() |
| 89 | + if stk: |
| 90 | + g[i].append(stk[-1]) |
| 91 | + stk.append(i) |
| 92 | + |
| 93 | + f = [inf] * n |
| 94 | + f[0] = 0 |
| 95 | + for i in range(n): |
| 96 | + for j in g[i]: |
| 97 | + f[j] = min(f[j], f[i] + costs[j]) |
| 98 | + return f[n - 1] |
66 | 99 | ```
|
67 | 100 |
|
68 | 101 | ### **Java**
|
69 | 102 |
|
70 | 103 | <!-- 这里可写当前语言的特殊实现逻辑 -->
|
71 | 104 |
|
72 | 105 | ```java
|
| 106 | +class Solution { |
| 107 | + public long minCost(int[] nums, int[] costs) { |
| 108 | + int n = nums.length; |
| 109 | + List<Integer>[] g = new List[n]; |
| 110 | + Arrays.setAll(g, k -> new ArrayList<>()); |
| 111 | + Deque<Integer> stk = new ArrayDeque<>(); |
| 112 | + for (int i = n - 1; i >= 0; --i) { |
| 113 | + while (!stk.isEmpty() && nums[stk.peek()] < nums[i]) { |
| 114 | + stk.pop(); |
| 115 | + } |
| 116 | + if (!stk.isEmpty()) { |
| 117 | + g[i].add(stk.peek()); |
| 118 | + } |
| 119 | + stk.push(i); |
| 120 | + } |
| 121 | + stk.clear(); |
| 122 | + for (int i = n - 1; i >= 0; --i) { |
| 123 | + while (!stk.isEmpty() && nums[stk.peek()] >= nums[i]) { |
| 124 | + stk.pop(); |
| 125 | + } |
| 126 | + if (!stk.isEmpty()) { |
| 127 | + g[i].add(stk.peek()); |
| 128 | + } |
| 129 | + stk.push(i); |
| 130 | + } |
| 131 | + long[] f = new long[n]; |
| 132 | + Arrays.fill(f, 1L << 60); |
| 133 | + f[0] = 0; |
| 134 | + for (int i = 0; i < n; ++i) { |
| 135 | + for (int j : g[i]) { |
| 136 | + f[j] = Math.min(f[j], f[i] + costs[j]); |
| 137 | + } |
| 138 | + } |
| 139 | + return f[n - 1]; |
| 140 | + } |
| 141 | +} |
| 142 | +``` |
| 143 | + |
| 144 | +### **C++** |
| 145 | + |
| 146 | +```cpp |
| 147 | +class Solution { |
| 148 | +public: |
| 149 | + long long minCost(vector<int>& nums, vector<int>& costs) { |
| 150 | + int n = nums.size(); |
| 151 | + vector<int> g[n]; |
| 152 | + stack<int> stk; |
| 153 | + for (int i = n - 1; ~i; --i) { |
| 154 | + while (!stk.empty() && nums[stk.top()] < nums[i]) { |
| 155 | + stk.pop(); |
| 156 | + } |
| 157 | + if (!stk.empty()) { |
| 158 | + g[i].push_back(stk.top()); |
| 159 | + } |
| 160 | + stk.push(i); |
| 161 | + } |
| 162 | + stk = stack<int>(); |
| 163 | + for (int i = n - 1; ~i; --i) { |
| 164 | + while (!stk.empty() && nums[stk.top()] >= nums[i]) { |
| 165 | + stk.pop(); |
| 166 | + } |
| 167 | + if (!stk.empty()) { |
| 168 | + g[i].push_back(stk.top()); |
| 169 | + } |
| 170 | + stk.push(i); |
| 171 | + } |
| 172 | + vector<long long> f(n, 1e18); |
| 173 | + f[0] = 0; |
| 174 | + for (int i = 0; i < n; ++i) { |
| 175 | + for (int j : g[i]) { |
| 176 | + f[j] = min(f[j], f[i] + costs[j]); |
| 177 | + } |
| 178 | + } |
| 179 | + return f[n - 1]; |
| 180 | + } |
| 181 | +}; |
| 182 | +``` |
73 | 183 |
|
| 184 | +### **Go** |
| 185 | +
|
| 186 | +```go |
| 187 | +func minCost(nums []int, costs []int) int64 { |
| 188 | + n := len(nums) |
| 189 | + g := make([][]int, n) |
| 190 | + stk := []int{} |
| 191 | + for i := n - 1; i >= 0; i-- { |
| 192 | + for len(stk) > 0 && nums[stk[len(stk)-1]] < nums[i] { |
| 193 | + stk = stk[:len(stk)-1] |
| 194 | + } |
| 195 | + if len(stk) > 0 { |
| 196 | + g[i] = append(g[i], stk[len(stk)-1]) |
| 197 | + } |
| 198 | + stk = append(stk, i) |
| 199 | + } |
| 200 | + stk = []int{} |
| 201 | + for i := n - 1; i >= 0; i-- { |
| 202 | + for len(stk) > 0 && nums[stk[len(stk)-1]] >= nums[i] { |
| 203 | + stk = stk[:len(stk)-1] |
| 204 | + } |
| 205 | + if len(stk) > 0 { |
| 206 | + g[i] = append(g[i], stk[len(stk)-1]) |
| 207 | + } |
| 208 | + stk = append(stk, i) |
| 209 | + } |
| 210 | + f := make([]int64, n) |
| 211 | + for i := 1; i < n; i++ { |
| 212 | + f[i] = math.MaxInt64 |
| 213 | + } |
| 214 | + for i := 0; i < n; i++ { |
| 215 | + for _, j := range g[i] { |
| 216 | + f[j] = min(f[j], f[i]+int64(costs[j])) |
| 217 | + } |
| 218 | + } |
| 219 | + return f[n-1] |
| 220 | +} |
| 221 | +
|
| 222 | +func min(a, b int64) int64 { |
| 223 | + if a < b { |
| 224 | + return a |
| 225 | + } |
| 226 | + return b |
| 227 | +} |
74 | 228 | ```
|
75 | 229 |
|
76 | 230 | ### **TypeScript**
|
77 | 231 |
|
78 | 232 | ```ts
|
79 |
| - |
| 233 | +function minCost(nums: number[], costs: number[]): number { |
| 234 | + const n = nums.length; |
| 235 | + const g: number[][] = Array.from({ length: n }, () => []); |
| 236 | + const stk: number[] = []; |
| 237 | + for (let i = n - 1; i >= 0; --i) { |
| 238 | + while (stk.length && nums[stk[stk.length - 1]] < nums[i]) { |
| 239 | + stk.pop(); |
| 240 | + } |
| 241 | + if (stk.length) { |
| 242 | + g[i].push(stk[stk.length - 1]); |
| 243 | + } |
| 244 | + stk.push(i); |
| 245 | + } |
| 246 | + stk.length = 0; |
| 247 | + for (let i = n - 1; i >= 0; --i) { |
| 248 | + while (stk.length && nums[stk[stk.length - 1]] >= nums[i]) { |
| 249 | + stk.pop(); |
| 250 | + } |
| 251 | + if (stk.length) { |
| 252 | + g[i].push(stk[stk.length - 1]); |
| 253 | + } |
| 254 | + stk.push(i); |
| 255 | + } |
| 256 | + const f: number[] = Array.from({ length: n }, () => Infinity); |
| 257 | + f[0] = 0; |
| 258 | + for (let i = 0; i < n; ++i) { |
| 259 | + for (const j of g[i]) { |
| 260 | + f[j] = Math.min(f[j], f[i] + costs[j]); |
| 261 | + } |
| 262 | + } |
| 263 | + return f[n - 1]; |
| 264 | +} |
80 | 265 | ```
|
81 | 266 |
|
82 | 267 | ### **...**
|
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