@@ -29,43 +29,48 @@ class Solution
2929 // Calculate every building point to landing point distance, and get the minm.
3030 int m = grid.size (), n = grid[0 ].size ();
3131 vector<int > direction = {-1 , 0 , 1 , 0 , -1 }; // left up right down
32- auto total = grid;
32+ vector<vector<int >> distance (m, vector<int >(n, 0 ));
33+ 3334 int min_dist = 0 , walk = 0 ;
3435 for (int i = 0 ; i < m; i++)
3536 {
3637 for (int j = 0 ; j < n; j++)
3738 {
3839 if (grid[i][j] == 1 )
3940 {
40- min_dist = - 1 ;
41- auto dist = grid ;
41+ // Note this is initialized to -1, so when a building is not reachable it will be -1 at the end
42+ min_dist = INT_MAX ;
4243 queue<pair<int , int >> que;
4344
4445 que.emplace (i, j);
45- 46+ int level = 1 ;
4647 while (!que.empty ())
4748 {
48- auto land = que.front ();
49- que.pop ();
49+ int len = que.size ();
5050
51- for (int k = 0 ; k < 4 ; k++)
51+ for (int k = 0 ; k < len ; k++)
5252 {
53- int x = land. first + direction[k] ;
54- int y = land. second + direction[k + 1 ] ;
55- if (x >= 0 && x < m && y > = 0 && y < n && walk == grid[x][y] )
53+ auto curr = que. front () ;
54+ que. pop () ;
55+ for ( int l = 0 ; l < 4 ; l++ )
5656 {
57- grid[x][y]--;
58- dist[x][y] = dist[land.first ][land.second ] + 1 ;
59- total[x][y] += dist[x][y] - 1 ;
60- que.emplace (x, y);
61- if (min_dist < 0 || min_dist > total[x][y])
57+ int x = curr.first + direction[l];
58+ int y = curr.second + direction[l + 1 ];
59+ if (x >= 0 && x < m && y >= 0 && y < n && walk == grid[x][y])
6260 {
63- min_dist = total[x][y];
61+ grid[x][y]--;
62+ distance[x][y] += level;
63+ 64+ que.emplace (x, y);
65+ min_dist = min (min_dist, distance[x][y]);
6466 }
6567 }
6668 }
69+ level++;
6770 }
6871 walk--;
72+ if (min_dist == INT_MAX)
73+ return -1 ;
6974 }
7075 }
7176 }
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