训练[2]-DFS

题目A:

题目B【https://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&page=show_problem&problem=614】

题意：

You are given a string consisting of parentheses () and []. A string of this type is said to be correct:

(a) if it is the empty string

(b) if A and B are correct, AB is correct,

Write a program that takes a sequence of strings of this type and check their correctness. Your program can assume that the maximum string length is 128.

题解：虽然长度最长只有128，但是是多组输入，用区间DP会TLE。简单stack的应用。

#include<stack>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
const int INF = 1e6;
const int MAXN = 150;
char S[MAXN];
int T, N;
int main ()
{
    scanf("%d", &T);
    getchar();
    while(T--)
    {
        gets(S + 1);
        N = strlen(S + 1);
        if(S[1] == ' ')//第一种情况
        {
            printf("Yes
");
            continue;
        }
        stack<char>st;
        for(int i = 1; i <= N; i++)
        {
            if(!st.empty() && ((st.top() == '(' && S[i] == ')') || (st.top() == '[' && S[i] == ']')))
                st.pop();//匹配
            else st.push(S[i]);
        }
        if(st.empty())    printf("Yes
");
        else    printf("No
");
    }
    return 0;
}

题目E【http://poj.org/problem?id=2386】

题意：给出一个图，mp[i][j]=='.'表示该地为干，mp[i][j]='W'表示该地是水坑，两个水坑联通的条件是八个方向任意一个方向联通则联通。求不联通水坑的个数。

题解：DFS,DFS的次数表示非联通水坑的数量。

#include<stack>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
typedef long long LL;
const int MAXN = 150;
char mp[MAXN][MAXN];
int N, M;
void DFS(int x, int y)
{
    mp[x][y] = '.';
    for(int i = -1; i <= 1; i++)
        for(int j = -1; j <= 1; j++)
            if(mp[x+i][y+j] == 'W')
                DFS(x+i, y+j);
}
int main ()
{
    int ans = 0;
    scanf("%d%d", &N, &M);
    for(int i = 1; i <= N; i++)
        scanf("%s", mp[i] + 1);
    for(int i = 1; i <= N; i++)
        for(int j = 1; j <= M; j++)
            if(mp[i][j] == 'W') DFS(i, j), ans++;
    printf("%d
", ans);
}

想的太多，做的太少。