LeetCode - 778 水位上升的泳池中游泳

class Solution {
    int N;
    public static final int[][] DIRECTIONS = {{0, 1}, {1, 0}, {-1, 0}, {0, -1}};
    public int swimInWater(int[][] grid) {
      N = grid.length;
      int[] indexs = new int[N*N]; 
      for(int i =0;i<N;i++){
          for(int j=0;j<N;j++){
              indexs[grid[i][j]] = getIndex(i,j);
          }
      }
      UnionFind uf = new UnionFind(N*N);
      int t = 0;
      while(uf.isConnect(getIndex(0,0),getIndex(N-1,N-1)) == false){
          int index = indexs[t];
          int x = index / N;
          int y = index % N;
          for(int[] direction:DIRECTIONS){
              int newX = x + direction[0];
              int newY = y + direction[1];
              if(inArea(newX,newY) && grid[newX][newY] < grid[x][y])
                  uf.union(getIndex(newX,newY),getIndex(x,y));
          }
          t += 1;
      }
      return Math.max(0,t-1);
    }
    public boolean inArea(int x,int y){
        return x >=0 && x < N && y >= 0 && y < N;
    }
    // public boolean inArea(int x,int y){
    //     return x>
    // }
    public int getIndex(int x,int y){
        return x * N + y;
    }
    public class UnionFind{
        int[] parent;
        int[] rank;
        public UnionFind(int n){
            parent = new int[n];
            rank = new int[n];
            for(int i = 0;i < n;i++){
                parent[i] = i;
                rank[i] = 1;
            }
        }
        public int find(int x){
            if(parent[x] != x){
                parent[x] = find(parent[x]);
            }
            return parent[x];
        }
        public boolean isConnect(int x,int y){
            int rootX = find(x);
            int rootY = find(y);
            if(rootX == rootY)
               return true;
            else
               return false;
        }
        public void union(int x,int y){
            int rootX = find(x);
            int rootY = find(y);
            if(rootX == rootY)
               return ;
            if(rank[rootX] == rank[rootY]){
                parent[rootX] = rootY;
                rank[rootY] += 1;
            }
            else if(rank[rootX] > rank[rootY]){
                parent[rootY] = rootX;
            }
            else{
                parent[rootX] = rootY;
            }
        }
    }
}