Solved problem 25. Min Ends Subsequence from CS Academy

Author: AliOsm

CS Academy/25. Min Ends Subsequence.cpp

@@ -0,0 +1,86 @@
+/*
+  Idea:
+    - Segment tree.
+    - Try each element with the maximum element after it
+      and count the number of elements between them less
+      than the current element.
+*/
+
+#include <bits/stdc++.h>
+
+using namespace std;
+
+int const N = 1e5 + 1;
+int n, a[N], mx[N];
+vector<int> seg[4 * N];
+
+void build(int at, int l, int r) {
+  if(l == r) {
+    seg[at].push_back(a[l]);
+    return;
+  }
+  
+  int mid = (l + r) >> 1;
+  build(at << 1, l, mid);
+  build(at << 1 | 1, mid + 1, r);
+  
+  int i = 0, j = 0;
+  for(; i < seg[at << 1].size() && j < seg[at << 1 | 1].size();)
+    if(seg[at << 1][i] < seg[at << 1 | 1][j])
+      seg[at].push_back(seg[at << 1][i++]);
+    else
+      seg[at].push_back(seg[at << 1 | 1][j++]);
+  while(i < seg[at << 1].size())
+    seg[at].push_back(seg[at << 1][i++]);
+  while(j < seg[at << 1 | 1].size())
+    seg[at].push_back(seg[at << 1 | 1][j++]);
+}
+
+int s, e, tar;
+int get(int at, int l, int r) {
+  if(l > e || r < s)
+    return 0;
+  
+  if(l >= s && r <= e)
+    return upper_bound(seg[at].begin(), seg[at].end(), tar) - seg[at].begin();
+  
+  int mid = (l + r) >> 1;
+  return get(at << 1, l, mid) + get(at << 1 | 1, mid + 1, r);
+}
+
+int solve() {
+  memset(mx, -1, sizeof mx);
+  for(int i = n, cur = 1; i > 0; --i)
+    while(cur <= a[i])
+      mx[cur++] = i;
+  
+  for(int i = 0; i < 4 * N; ++i)
+    seg[i].clear();
+  build(1, 1, n);
+  
+  int ret = 0;
+  for(int i = 1; i <= n; ++i) {
+    s = i, e = mx[a[i]], tar = a[i];
+    ret = max(ret, get(1, 1, n) + 1);
+  }
+  
+  return ret;
+}
+
+int main() {
+  scanf("%d", &n);
+  for(int i = 1; i <= n; ++i)
+    scanf("%d", a + i);
+  
+  if(n == 1) {
+    puts("1");
+    return 0;
+  }
+  
+  int r1 = solve();
+  reverse(a + 1, a + n + 1);
+  int r2 = solve();
+  printf("%d\n", max(r1, r2));
+  
+  return 0;
+}

CS Academy/README.md

@@ -4,6 +4,7 @@
 - [18. Consecutive Digits Signs](https://csacademy.com/contest/round-18/task/consecutive-digit-signs/)
 - [24. Vector Size](https://csacademy.com/contest/round-24/#task/vector-size)
 - [24. Kth Special Number](https://csacademy.com/contest/round-24/#task/kth-special-number)
+- [25. Min Ends Subsequence](https://csacademy.com/contest/round-25/task/min-ends-subsequence)
 - [26. Limited Vocabulary](https://csacademy.com/contest/round-26/#task/limited-vocabulary)
 - [26. Odd Pair Sums](https://csacademy.com/contest/round-26/#task/odd-pair-sums)
 - [27. Backpack Packing](https://csacademy.com/contest/round-27/#task/backpack-packing)