leetcode[163] Missing Ranges

string& makeRange(int lo, int hi){
    static string result;
    result="";
    stringstream ss;
    if (lo != hi){
        ss << lo << "->" << hi;
    }else{
        ss << lo;
    }
    ss >> result;
    return result;
}


vector<string> findMissingRanges(int A[], int n, int lower, int upper) {

    vector<string> result;

    if ( n<=0 ) {
        result.push_back(makeRange(lower, upper));
        return result;
    }

    if (lower < A[0]){
        result.push_back(makeRange(lower, A[0]-1 < upper ? A[0]-1 : upper));
    }

    for(int i=0; i<n-1; i++){
        if ( A[i] + 1 == A[i+1] ) {
            continue;
        }
        result.push_back(makeRange(A[i]+1, A[i+1]-1));
    }

    if (upper > A[n-1]){
        result.push_back(makeRange(A[n-1]+1, upper));
    }

    return result;
}