HDU 2859—Phalanx(DP)
Time Limit:5000MS Memory Limit:32768KB 64bit IO Format:%I64d & %I64u
Description
Today is army day, but the servicemen are busy with the phalanx for the celebration of the 60th anniversary of the PRC.
A phalanx is a matrix of size n*n, each element is a character (a~z or A~Z), standing for the military branch of the servicemen on that position.
For some special requirement it has to find out the size of the max symmetrical sub-array. And with no doubt, the Central Military Committee gave this task to ALPCs.
A symmetrical matrix is such a matrix that it is symmetrical by the “left-down to right-up” line. The element on the corresponding place should be the same. For example, here is a 3*3 symmetrical matrix:
cbx
cpb
zcc
Input
There are several test cases in the input file. Each case starts with an integer n (0<n<=1000), followed by n lines which has n character. There won’t be any blank spaces between characters or the end of line. The input file is ended with a 0.
Output
Each test case output one line, the size of the maximum symmetrical sub- matrix.
Sample Input
3
abx
cyb
zca
4
zaba
cbab
abbc
cacq
0
Sample Output
3
3 大意: 给定一个矩阵,求最大对称矩阵 思路: 一开始怎么想也觉得不会有状态转移的情况,即使有也会复杂度过高 后来看了题解才想通,其实和我一开始的想的有些相似,因为给的时间比较长,n^3的复杂度也会死可以接受的 试想一个n阶的对称阵如何变成n+1的对称阵? 在它的两个底边追加对称的元素即可,对角线元素添加任意元素即可 于是,我们从一个点向他的上与右边推进,直至不匹配 将匹配的个数与dp[i-1][j+1]比较, 如果匹配量大于右上角记录下来的矩阵大小,就是右上角的数值+1,否则就是这个匹配量。 代码:#include<bits/stdc++.h>
using namespace std;
const int MAXN=1300;
char m[MAXN][MAXN];
int dp[MAXN][MAXN];
int main()
{
//freopen("data.in","r",stdin);
int n;
while(~scanf("%d",&n)&&n){
getchar();
for(int i=0;i<n;i++){
for(int j=0;j<n;j++){
scanf("%c",&m[i][j]);
}
getchar();
}
// for(int i=0;i<n;i++){
// for(int j=0;j<n;j++){
// cout<<m[i][j];
// }
// }
// cout<<endl;
memset(dp,0,sizeof(dp));
int res=1;
for(int i=0;i<n;i++){
for(int j=n-1;j>=0;j--){
//cout<<i<<"\t"<<j<<"\t";
//cout<<1111111<<endl;
if(i==0||j==n-1){
dp[i][j]=1;
//cout<<"is "<<dp[i][j]<<endl;
continue;
}
int x=i,y=j;
while(x>=0&&y<=n-1&&m[x][j]==m[i][y]){
x--;
y++;
}
y=y-j;
if(y>dp[i-1][j+1]){
dp[i][j]=dp[i-1][j+1]+1;
}
else{
dp[i][j]=y;
}
//cout<<"is "<<dp[i][j]<<endl;
res=max(res,dp[i][j]);
}
}
printf("%d\n",res);
}
}
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