Create 200. Number of Islands.md

solutions/200. Number of Islands.md

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+# [200. Number of Islands](https://leetcode.com/problems/number-of-islands/)
+# 思路
+求岛屿数量. 简单用DFS或者BFS就可以了. 下面代码用的DFS.
+
+> 亲测visited数组定义成bool型要比定义成int型慢不少.
+
+# C++
+``` C++
+class Solution {
+    private:
+    vector<int>dx{-1, 1, 0, 0}; // 上下左右四个方向
+    vector<int>dy{0, 0, -1, 1};
+    void DFS(int i, int j, const vector<vector<char>>& grid, vector< vector<int>>& visited){
+        if(i < 0 || i >= grid.size() || j < 0 || j >= grid[0].size()) return;
+        if(visited[i][j] || grid[i][j] == '0') return;
+        
+        visited[i][j] = 1;
+        for(int k = 0; k < 4; k++)
+            DFS(i + dx[k], j + dy[k], grid, visited);
+    }
+public:
+    int numIslands(vector<vector<char>>& grid) {
+        int m = grid.size(), res = 0;
+        if(!m) return 0;
+        int n = grid[0].size();
+        
+        vector< vector<int>>visited(m, vector<int>(n, 0)); // int比bool型数组快
+        for(int i = 0; i < m; i++){
+            for(int j = 0; j < n; j++){
+                if(visited[i][j] || grid[i][j] == '0') continue;
+                DFS(i, j, grid, visited);
+                res++;
+            }
+        }
+        return res;
+    }
+};
+```