问(dart)place函数保存St初始状态的Keepst里的内容会随St改变比如n1k1i0j3时？

//place，里的保存初始状态的Keepst，不知道为什么里面的内容总是会随着St改变，怎么都不行
class Solution {
  int guardCastle(List<String> grid) {
    List<String> st = grid[0].split(''),
        nd = grid[1].split(''),
        keepst = List.from(st),
        keepnd = List.from(nd); //初始化
    int answer = -2;
    grid=move(List.from(st), List.from(nd));//函数传参不能传参数本身，只传值List<String>.from()
    st=grid[0].split('');
    nd=grid[1].split('');//无放置，更新数据
    if (st.contains('C') || nd.contains('C'))
      return 0;
    else {
      st = keepst;
      nd = keepnd;
    }
    for (int i = 1; i < grid[0].length; i++) {
      //放置
      answer = place(List.from(st),List.from(nd), i, 1);
      if (answer > 0 || i + 1 == grid[0].length) return answer;
    }
    return answer;
  }

  List<String> move(List<String> St, List<String> Nd) {
    for (int i = 0; i < St.length; i++) {
      //往下移动
      if (St[i] == 'S' && Nd[i] != '#' && Nd[i] != 'S') {
        if (Nd[i] == 'P') {
          for (int j = 0; j < St.length; j++) {
            if (St[j] == 'P') St[j] = 'S';
            if (Nd[j] == 'P') Nd[j] = 'S';
          }
        }
        Nd[i] = 'S';
        i = -1;
        //往上移动
      } else if (Nd[i] == 'S' && St[i] != '#' && St[i] != 'S') {
        if (St[i] == 'P') {
          for (int j = 0; j < St.length; j++) {
            if (St[j] == 'P') St[j] = 'S';
            if (Nd[j] == 'P') Nd[j] = 'S';
          }
        }
        St[i] = 'S';
        i = -1;
        //第一行往右移动
      } else if (i < St.length - 1 &&
          St[i] == 'S' &&
          St[i + 1] != '#' &&
          St[i + 1] != 'S') {
        if (St[i + 1] == 'P') {
          for (int j = 0; j < St.length; j++) {
            if (St[j] == 'P') St[j] = 'S';
            if (Nd[j] == 'P') Nd[j] = 'S';
          }
        }
        St[i + 1] = 'S';
        i = -1;
        //第一行往左移动
      } else if (i > 0 &&
          St[i] == 'S' &&
          St[i - 1] != '#' &&
          St[i - 1] != 'S') {
        if (St[i - 1] == 'P') {
          for (int j = 0; j < St.length; j++) {
            if (St[j] == 'P') St[j] = 'S';
            if (Nd[j] == 'P') Nd[j] = 'S';
          }
        }
        St[i - 1] = 'S';
        i = -1;
        //第二行往右移动
      } else if (i < Nd.length - 1 &&
          Nd[i] == 'S' &&
          Nd[i + 1] != '#' &&
          Nd[i + 1] != 'S') {
        if (Nd[i + 1] == 'P') {
          for (int j = 0; j < St.length; j++) {
            if (St[j] == 'P') St[j] = 'S';
            if (Nd[j] == 'P') Nd[j] = 'S';
          }
        }
        Nd[i + 1] = 'S';
        i = -1;
        //第二行往左移动
      } else if (i > 0 &&
          Nd[i] == 'S' &&
          Nd[i - 1] != '#' &&
          Nd[i - 1] != 'S') {
        if (Nd[i - 1] == 'P') {
          for (int j = 0; j < St.length; j++) {
            if (St[j] == 'P') St[j] = 'S';
            if (Nd[j] == 'P') Nd[j] = 'S';
          }
        }
        Nd[i - 1] = 'S';
        i = -1;
      }
    }
    List<String> Grid=[St.join(),Nd.join()];
    return Grid;
  }

  int place(List<String> St,List<String> Nd, int n, int k) {
    List<String> Keepst = List.from(St), Keepnd = List.from(Nd); //初始化
    List<String> Grid = [St.join(),Nd.join()];
    int Answer = -3;
    for (int i = 0; i < 2; i++) {
      for (int j = 0; j < Grid[0].length; j++) {
        if (i == 0 && Grid[i][j] == '.') {
          //第一行
          St[j] = '#';
          if (k < n) {
            Answer = place(List.from(St),List.from(Nd), n, k++);
          }
          if (Answer > 0) return Answer;
          Grid=move(List.from(St), List.from(Nd));
          St=Grid[0].split('');
          Nd=Grid[1].split('');
          if (St.contains('C') || Nd.contains('C'))
            return k;
          else {
            St = Keepst;
            Nd = Keepnd;
            Grid = [St.join(),Nd.join()];
          }
        } else if (i == 1 && Grid[i][j] == '.') {
          //第二行
          Nd[j] = '#';
          if (k < n) {
            Answer = place(List.from(St),List.from(Nd), n, k++);
          }
          if (Answer > 0) return Answer;
          Grid=move(List.from(St), List.from(Nd));
          St=Grid[0].split('');
          Nd=Grid[1].split('');
          if (St.contains('C') || Nd.contains('C'))
            return k;
          else {
            St = Keepst;
            Nd = Keepnd;
            Grid = [St.join(),Nd.join()];
          }
        }
        if (i == 1 && j + 1 == Grid[0].length) return -1;
      }
    }
    return -3;
  }
}

void main() {
  List<String> grid = ["S.C.P#P.", ".....#.S"];
  Solution solution = Solution();
  int result = solution.guardCastle(grid);
  print(result);
}

          St[j] = '#';
          if (k < n) {