[LeetCode] 317. Shortest Distance from All Buildings 建筑物的最短距离
2018-08-29 04:20
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You want to build a house on an empty land which reaches all buildings in the shortest amount of distance. You can only move up, down, left and right. You are given a 2D grid of values 0, 1 or 2, where:
- Each 0 marks an empty land which you can pass by freely.
- Each 1 marks a building which you cannot pass through.
- Each 2 marks an obstacle which you cannot pass through.
For example, given three buildings at
(0,0),
(0,4),
(2,2), and an obstacle at
(0,2):
1 - 0 - 2 - 0 - 1 | | | | | 0 - 0 - 0 - 0 - 0 | | | | | 0 - 0 - 1 - 0 - 0
The point
(1,2)is an ideal empty land to build a house, as the total travel distance of 3+3+1=7 is minimal. So return 7.
Note:
There will be at least one building. If it is not possible to build such house according to the above rules, return -1.
给一个2纬网格,0代表空地可自由通过,1代表建筑物不能通过,2代表障碍物不可通过,找一个位置建房子,使其到所有建筑物的曼哈顿距离之和最小。返回建房子的位置,如果没有这样的位置返回-1。
解法:BFS
Python:
# Time: O(k * m * n), k is the number of the buildings # Space: O(m * n) class Solution(object): def shortestDistance(self, grid): """ :type grid: List[List[int]] :rtype: int """ def bfs(grid, dists, cnts, x, y): dist, m, n = 0, len(grid), len(grid[0]) visited = [[False for _ in xrange(n)] for _ in xrange(m)] pre_level = [(x, y)] visited[x][y] = True while pre_level: dist += 1 cur_level = [] for i, j in pre_level: for dir in [(-1, 0), (1, 0), (0, -1), (0, 1)]: I, J = i+dir[0], j+dir[1] if 0 <= I < m and 0 <= J < n and grid[I][J] == 0 and not visited[I][J]: cnts[I][J] += 1 dists[I][J] += dist cur_level.append((I, J)) visited[I][J] = True pre_level = cur_level m, n, cnt = len(grid), len(grid[0]), 0 dists = [[0 for _ in xrange(n)] for _ in xrange(m)] cnts = [[0 for _ in xrange(n)] for _ in xrange(m)] for i in xrange(m): for j in xrange(n): if grid[i][j] == 1: cnt += 1 bfs(grid, dists, cnts, i, j) shortest = float("inf") for i in xrange(m): for j in xrange(n): if dists[i][j] < shortest and cnts[i][j] == cnt: shortest = dists[i][j] return shortest if shortest != float("inf") else -1
C++:
class Solution { public: int shortestDistance(vector<vector<int>>& grid) { int res = INT_MAX, val = 0, m = grid.size(), n = grid[0].size(); vector<vector<int>> sum = grid; vector<vector<int>> dirs{{0,-1},{-1,0},{0,1},{1,0}}; for (int i = 0; i < grid.size(); ++i) { for (int j = 0; j < grid[i].size(); ++j) { if (grid[i][j] == 1) { res = INT_MAX; vector<vector<int>> dist = grid; queue<pair<int, int>> q; q.push({i, j}); while (!q.empty()) { int a = q.front().first, b = q.front().second; q.pop(); for (int k = 0; k < dirs.size(); ++k) { int x = a + dirs[k][0], y = b + dirs[k][1]; if (x >= 0 && x < m && y >= 0 && y < n && grid[x][y] == val) { --grid[x][y]; dist[x][y] = dist[a][b] + 1; sum[x][y] += dist[x][y] - 1; q.push({x, y}); res = min(res, sum[x][y]); } } } --val; } } } return res == INT_MAX ? -1 : res; } };
C++:
class Solution { public: int shortestDistance(vector<vector<int>>& grid) { int res = INT_MAX, buildingCnt = 0, m = grid.size(), n = grid[0].size(); vector<vector<int>> dist(m, vector<int>(n, 0)), cnt = dist; vector<vector<int>> dirs{{0,-1},{-1,0},{0,1},{1,0}}; for (int i = 0; i < m; ++i) { for (int j = 0; j < n; ++j) { if (grid[i][j] == 1) { ++buildingCnt; queue<pair<int, int>> q; q.push({i, j}); vector<vector<bool>> visited(m, vector<bool>(n, false)); int level = 1; while (!q.empty()) { int size = q.size(); for (int s = 0; s < size; ++s) { int a = q.front().first, b = q.front().second; q.pop(); for (int k = 0; k < dirs.size(); ++k) { int x = a + dirs[k][0], y = b + dirs[k][1]; if (x >= 0 && x < m && y >= 0 && y < n && grid[x][y] == 0 && !visited[x][y]) { dist[x][y] += level; ++cnt[x][y]; visited[x][y] = true; q.push({x, y}); } } } ++level; } } } } for (int i = 0; i < m; ++i) { for (int j = 0; j < n; ++j) { if (grid[i][j] == 0 && cnt[i][j] == buildingCnt) { res = min(res, dist[i][j]); } } } return res == INT_MAX ? -1 : res; } };
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