xieqilu
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+/**
+Problem Statement
+
+You are given a square map of size n×n. Each cell of the map has a value denoting its depth. We will call a cell of the map 
+a cavity if and only if this cell is not on the border of the map and each cell adjacent to it has strictly smaller depth. 
+Two cells are adjacent if they have a common side.
+
+You need to find all the cavities on the map and depict them with the uppercase character X.
+
+The first line contains an integer, n, denoting the size of the map. Each of the following n lines contains n positive digits without spaces.
+Each digit (1-9) denotes the depth of the appropriate area.
+
+Constraints
+1≤n≤100
+
+Output Format
+Output n lines, denoting the resulting map. Each cavity should be replaced with character X.
+
+Sample Input
+4
+1112
+1912
+1892
+1234
+Sample Output
+1112
+1X12
+18X2
+1234
+
+
+Solution:
+This is a very simple problem, but there are some points to be noticed. The method is just traverse the whole map, if current
+cell is a border cell, output its value. Then compare current cell with all four adjecent cells, if the value of current cell is
+larger than all four adjecent cells, output "X". Otherwise output its value.
+
+Note:
+1, When parse the input and build a 2D int array (matrix), the line of input doesn't contain any space. So converse the line to
+a char array using string.ToCharArray(), then map[i,j] = line[j]-'0'. Don't try to use int.Parse to convert a string array to int.
+Think outside of the box.
+
+2, Don't try to modify the value of the matrix then output the whole matrix. This will require an extra traverse which is not
+efficient. Besides, we cannot change the value of an integer cell to "X", it's better to output the result when traversing the
+matrix. Then we only need to traverse for once.
+*/
+
+using System;
+using System.Collections.Generic;
+using System.IO;
+
+class Solution {
+    public static void FindCavity(int[,] map, int size){
+        for(int i=0;i<size;i++){
+            for(int j=0;j<size;j++){
+                if(i==0 || j==0 || i==size-1 || j==size-1)
+                    Console.Write(map[i,j]);
+                else if(map[i,j]>map[i-1,j]&&map[i,j]>map[i+1,j]&&map[i,j]>map[i,j+1]&&map[i,j]>map[i,j-1])
+                    Console.Write("X");
+                else
+                    Console.Write(map[i,j]);
+            }
+            Console.WriteLine(); //start a new line
+        }
+    }
+    
+    static void Main(String[] args) {
+        /* Enter your code here. Read input from STDIN. Print output to STDOUT. Your class should be named Solution */
+        int size = int.Parse(Console.ReadLine());
+        int[,] map = new int[size,size];
+        for(int i=0;i<size;i++){
+            char[] line = Console.ReadLine().ToCharArray();
+            for(int j=0;j<size;j++)
+                map[i,j] = line[j]-'0';
+        }
+        FindCavity(map,size);
+    }
+}