class Solution {
public:
    bool validMountainArray(vector<int>& A) {
        int n=A.size(),x=0;
        for(;x<n-2&&A[x+1]>A[x];x++);
        if(!x) return 0;
        for(;x<n-1&&A[x+1]<A[x];x++);
        return x==n-1;
    }
};