uva10534

make a 2*l sequence which is + in 0-l-1 and - in l-2*l-1, and make sure the l is max

main algorithm

keep a up-going sequence from 0 - n-1 and n-1 - 0, so the d1[i] is the length of up-going sequence from 0 - i-1 and d2[i] is the down-going sequence of i-n-1 

code:

#include<cstdio>#include<cstdlib>#include<algorithm>#include<iostream>#include<climits>using namespace std;const int N = 10005;int stack[N], d1[N], d2[N];int a[N];int search(int l, int r, int x) {while(l<=r) {int mid = (l+r)/2;if(x > stack[mid])l = mid+1;else r = mid-1;}return l;}int n;int main() {while(scanf("%d", &n) == 1) {for(int i=1; i<=n; i++) scanf("%d", &a[i]);int top = 0;stack[top] = INT_MIN;for(int i=1; i<=n; i++) {if(a[i] > stack[top]) stack[++top] = a[i];else {int tmp = search(1, top, a[i]);stack[tmp] = a[i];}d1[i] = top;}top = 0, stack[0] = INT_MIN;for(int i=n; i>=1; i--) {if(a[i] > stack[top]) stack[++top] = a[i];else {int tmp = search(1, top, a[i]);stack[tmp] = a[i];}d2[i] = top;}int ans = 0;for(int i=1; i<=n; i++) {if(ans<min(d1[i], d2[i])) ans = min(d1[i], d2[i]);}printf("%d\n", 2*ans-1);}return 0;}