C++ 中的名称冲突之 "y1"

已经是第二次遇到这个问题了：

#include <bits/stdc++.h>
using namespace std;
 
 
const int N(105);
 
int dp[N][2][N][2][N];
int x1, x2, y1, y2;
 
int dfs(int x1, int s, int x2, int t, int y){
    int & now=dp[x1][s][x2][t][y];
    if(~now) return now;
    int sum=0, cnt=0;
    if(s==0){
        sum+=dfs(x1, 1, x2, t, y-1), cnt++;
        if(t==0)
            sum+=dfs(x1, s, x2, 1, y-1), cnt++;
        else if(x2>0) 
            sum+=dfs(x1, 0, x2-1, t, y), cnt++;
    }
    else{
        if(x1>0) sum+=dfs(x1-1, s, x2, t, y), cnt++;
        if(t==0)
            sum+=dfs(x1, s, x2, 1, y-1), cnt++;
        else if(x2>0)
            sum+=dfs(x1, s, x2-1, t, y), cnt++;
    }

    if((y1+1)*(1-s)+(y2+1)*(1-t)< y) //forbidden numbers;
        sum+=dfs(x1, s, x2, t, y-1), cnt++;
    
    return now = sum!=cnt;
}
 
int a[N];

int main(){
    int T;
    int n;
    for(cin>>T; T--; ){
        cin>>n;
        int p;
        for(int i=0; i<n; i++){
            cin>>a[i];
            if(a[i]==1) p=i;
        }
        x1=0, x2=0, y1=0, y2=0;
        int i;

        for(i=p-1; i>=0 && a[i]>a[i+1]; i--, x1++);

        for(; i>=0 && a[i]<a[i+1]; i--, y1++);
        for(i=p+1; i<n && a[i]>a[i-1]; i++, x2++);
        for(; i<n && a[i]<a[i-1]; i++, y2++);

        memset(dp, -1, sizeof(dp));
        x1=max(x1-1, 0), x2=max(x2-1, 0);

        // cout<<x1<<' '<<x2<<' '<<y1<<' '<<y2<<endl;
        int peak=(p!=0) + (p!=n-1);
        for(int i=0; i<=n-1-x1-x2-peak; i++)
            dp[0][1][0][1][i]=1;    //error-prone

        int s=p==0, t=p==n-1;   //error-prone

        int res=dfs(x1, s, x2, t, n-1-x1-x2);
        // cout<<res<<endl;
        puts(res==1?"Alice": "Bob");
    }
    return 0;
}

试图在终端编译，运行：　(gcc version 4.8.4)

g++ main.cpp -std=c++11 -o main && ./main <in

返回结果：

main.cpp:8:13: error: ‘int y1’ redeclared as different kind of symbol
 int x1, x2, y1, y2;
             ^
In file included from /usr/include/features.h:374:0,
                 from /usr/include/assert.h:35,
                 from /usr/include/c++/4.8/cassert:43,
                 from /usr/include/x86_64-linux-gnu/c++/4.8/bits/stdc++.h:33,
                 from main.cpp:1:
/usr/include/x86_64-linux-gnu/bits/mathcalls.h:241:1: error: previous declaration of ‘double y1(double)’
 __MATHCALL (y1,, (_Mdouble_));
 ^

显示变量 y1 和 C++ 标准库中的某个变量名称冲突，这个问题应当引起注意。

另外这不是头文件写成 <bits/stdc++.h> 引起的，即使换成各具体的头文件（<iostream>, <algorithm>, <ctring>）还是会发生这个错误。

具体原因及解决办法还有待研究。