http://codeforces.com/contest/846/problem/D

题解：二维RMQ问题+ST表

/*
*@Author:   STZG
*@Language: C++
*/
#include <bits/stdc++.h>
#include<iostream>
#include<algorithm>
#include<cstdlib>
#include<cstring>
#include<cstdio>
#include<string>
#include<vector>
#include<bitset>
#include<queue>
#include<deque>
#include<stack>
#include<cmath>
#include<list>
#include<map>
#include<set>
//#define DEBUG
#define RI register int
#define endl "\n"
using namespace std;
typedef long long ll;
//typedef __int128 lll;
const int N=500+10;
const int M=100000+10;
const int MOD=1e9+7;
const double PI = acos(-1.0);
const double EXP = 1E-8;
const int INF = 0x3f3f3f3f;
int t,n,m,k,q,l,r,u,v,x,y;
int ans,cnt,flag,temp,sum;
int a[N][N];
int dp[N][N];
int c[N][N];
char str;
struct node{};
void ST(int x){
    for(int i=1;i<=m;i++){
        dp[i][0]=a[x][i];
    }
    for(int j=0;(1<<j)<m;j++){
        for(int i=1;i+(1<<j)<=m;i++){
            dp[i][j+1]=max(dp[i][j],dp[i+(1<<j)][j]);

        }
    }
    for(int i=1;i<=m-k+1;i++)
        c[x][i]=max(dp[i][t],dp[i+k-(1<<t)][t]);
}
void ST2(int x){
    for(int i=1;i<=n;i++){
        dp[i][0]=c[i][x];
    }
    for(int j=0;(1<<j)<n;j++){
        for(int i=1;i+(1<<j)<=n;i++){
            dp[i][j+1]=max(dp[i][j],dp[i+(1<<j)][j]);
        }
    }
    for(int i=1;i<=n-k+1;i++)
        ans=min(ans,max(dp[i][t],dp[i+k-(1<<t)][t]));

}
int main()
{
#ifdef DEBUG
	freopen("input.in", "r", stdin);
	//freopen("output.out", "w", stdout);
#endif
    //ios::sync_with_stdio(false);
    //cin.tie(0);
    //cout.tie(0);
    //scanf("%d",&t);
    //while(t--){
    scanf("%d%d%d%d",&n,&m,&k,&q);
    memset(a,INF,sizeof(a));
    while(q--){
        scanf("%d%d%d",&x,&y,&t);
        a[x][y]=t;
    }
    t=log2(k);
    for(int i=1;i<=n;i++)ST(i);
    ans=INF;
    for(int i=1;i<=m-k+1;i++)ST2(i);
    cout<<(ans==INF?-1:ans)<<endl;
    //}

#ifdef DEBUG
	printf("Time cost : %lf s\n",(double)clock()/CLOCKS_PER_SEC);
#endif
    //cout << "Hello world!" << endl;
    return 0;
}