力扣778.水位上升的泳池中游泳

力扣778.水位上升的泳池中游泳

• 二分 + bfs

  class Solution {
      int dx[4] = {1,0,-1,0},dy[4] = {0,1,0,-1};
  public:
      int swimInWater(vector<vector<int>>& grid) {
          int n = grid.size();
          auto check = [&](int mid) -> bool
          {
              queue<pair<int,int>> q;
              vector<int> st(n*n);
              q.emplace(0,0);
              st[0] = 1;
              while(!q.empty())
              {
                  auto [x,y] = q.front();
                  q.pop();
                  for(int i=0;i<4;i++)
                  {
                      int a = x + dx[i];
                      int b = y + dy[i];
                      if(a >= 0 && a < n && b < n && b >= 0 && !st[a*n+b] && grid[a][b] <= mid)
                      {
                          q.emplace(a,b);
                          st[a*n+b] = 1;
                      }
                  }
              }
              if(st[n*n-1]) return true;
              else return false;
          };
  
          int l = grid[0][0] , r = 2501;
          while(l<r) 
          {
              int mid = l + r >> 1;
              if(check(mid)) r = mid;
              else l = mid + 1;
          }
          return l;
      }
  };