1272 最大距离 只想到了dp

http://www.51nod.com/onlineJudge/questionCode.html#!problemId=1272

离散化后，用dp[i]表示向右，大于等于i这个数字的最大位置

dp[i] = max(dp[i + 1], dp[i])

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <assert.h>
#define IOS ios::sync_with_stdio(false)
using namespace std;
#define inf (0x3f3f3f3f)
typedef long long int LL;

#include <iostream>
#include <sstream>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <string>
#include <bitset>
const int maxn = 50000 + 20;
int a[maxn], dp[maxn], id[maxn];
int n;
vector<int>da;
void work() {
    scanf("%d", &n);
    int top = 0;
    da.push_back(0);
    for (int i = 1; i <= n; ++i) {
        scanf("%d", &a[i]);
        da.push_back(a[i]);
    }
    sort(da.begin(), da.end());
    for (int i = 1; i <= n; ++i) {
        id[i] = lower_bound(da.begin(), da.end(), a[i]) - da.begin();
    }
    for (int i = n; i >= 1; --i) {
        dp[id[i]] = max(dp[id[i]], i);
    }
//    for (int i = 1; i <= n; ++i) {
//        cout << id[i] << " ";
//    }
    for (int i = n; i >= 1; --i) {
        dp[i] = max(dp[i], dp[i + 1]);
    }
    int ans = 0;
    for (int i = 1; i <= n; ++i) {
        ans = max(ans, dp[id[i]] - i);
    }
    cout << ans << endl;
}

int main() {
#ifdef local
    freopen("data.txt", "r", stdin);
//    freopen("data.txt", "w", stdout);
#endif
    work();
    return 0;
}

然后这题排个序，然后记录下最小的id就好。

把思维转过来，每个i，去右边找，相当于每个i，向左边找，一样的。