65 lines
1.9 KiB
C++
65 lines
1.9 KiB
C++
#include <iostream>
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using namespace std;
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using Segment = pair<int, int>;
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constexpr int MAX_N = 20005;
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int n;
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Segment segments[MAX_N];
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pair<int, int> dp[MAX_N]; // 从这一层的线段左/右边出去
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void log_dp() {
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for (int i = 0; i < n; i++) {
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clog << dp[i].first << "\t" << dp[i].second << endl;
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}
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}
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pair<int, int> get_min_steps(const Segment& segment, const int start_pos) {
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const int len = segment.second - segment.first;
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if (start_pos <= segment.first) {
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return {segment.second - start_pos + len, segment.second - start_pos};
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}
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if (start_pos >= segment.second) {
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return {start_pos - segment.first, start_pos - segment.first + len};
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}
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return {
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len + (segment.second - start_pos),
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len + (start_pos - segment.first)
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};
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}
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// pair<int, int> get_min_steps(const Segment& segment, const Segment& previous_segment) {
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// auto [first1, second1] = get_min_steps(segment, previous_segment.second);
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// auto [first2, second2] = get_min_steps(segment, previous_segment.first);
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// return {min(first1, first2), min(second1, second2)};
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// }
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int solve_dp() {
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dp[0] = get_min_steps(segments[0], 1);
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for (int i = 1; i < n; i++) {
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auto [first1, second1] = get_min_steps(segments[i], segments[i - 1].first);
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auto [first2, second2] = get_min_steps(segments[i], segments[i - 1].second);
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first1 += dp[i - 1].first, second1 += dp[i - 1].first;
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first2 += dp[i - 1].second, second2 += dp[i - 1].second;
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dp[i] = {min(first1, first2), min(second1, second2)};
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}
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return min(dp[n - 1].first, dp[n - 1].second);
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}
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int main() {
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ios::sync_with_stdio(false);
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cin.tie(nullptr);
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cin >> n;
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for (int i = 0; i < n; i++) {
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cin >> segments[i].first >> segments[i].second;
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}
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segments[n] = {n, n}, n++;
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cout << solve_dp() + n - 2 << endl;
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log_dp();
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return 0;
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}
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