Postman sort readme added (#2829)

Postman_Sort/readme.md

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+# Postman Sort
+
+Postman sort works by sorting the integers of an array from their most significant digits to their least 
+significant digits. In the Postman sort the integer having most significant digits and the number of
+elements in that integer is determined, the length of the longest integer is stored. All the elements in the 
+array are divided by a particular base. The elements of the array are sorted on the basis of the most significant
+digit to the least significant digit i.e. from leftmost to rightmost digit.
+
+## Example
+
+Let's arrange the numbers in ascending order using postman sort.
+
+- Unsorted List
+
+The input array contains 7 elements which are {25, 432, 788, 130, 23, 121, 564}
+
+- Iteration 1
+
+The elements of the array being sorted on the basis of the most significant digit, *it becomes {25, 23, 130, 121, 432, 564, 788}*
+
+- Iteration 2
+
+The elements of the array being sorted on the basis of the second most significant digit, *it becomes {25, 23, 121, 130, 432, 564, 788}*
+
+- Iteration 3
+
+At last those elements of the array are compared which have same value of significant digit and if they are not in correct 
+order, the elements gets swapped and are arranged in desired order. The array after getting sorted *becomes
+{23, 25, 121, 130, 432, 564, 788}*
+
+*In this way the elements of the array are sorted on the basis of their significant digits.*
+
+## Algorithm
+
+1.Compare the leftmost digit of the elements of the array then the next to it and so on.<br/>
+
+2.If the elements are not in correct order swap them according to their significant digit.<br/>
+
+3.The function recurses which compares the elements having same value of significant digit and arranges the elements in correct order.<br/>
+
+## Pseudocode
+```
+declare temp, max, count, maxdigits = 0, c = 0
+declare t1, t ,i, j
+Initialise n = 1
+Input the number of elements in the array and store their values in arr[], arr1[]
+for(i = 0; i < count; i++)
+    t = arr[i]
+    while(t > 0)
+        c++
+        t = t/10
+    if(maxdigits < c)
+        maxdigits = c
+    c = 0
+    
+for(i = 0; i < maxdigits; i++)
+    n = n * 10
+for(i = 0; i < count; i++)
+    max = arr[i] / n
+    t = i
+    for(j = i + 1; j < count; j++)
+        if(max > (arr[j] / n))
+            max = arr[j] / n
+            t = j       
+    temp = arr1[t]
+    arr1[t] = arr1[i]
+    arr1[i] = temp
+    temp = arr[t]
+    arr[t] = arr[i]
+    arr[i] = temp
+while(n >= 1)
+    for(i = 0; i < count;)
+        t1 = arr[i] / n
+        for(j = i + 1; t1 == (arr[j]/n); j++);
+            arrange(i, j)
+        i = j
+    n = n / 10
+              
+void arrange(int k, int n)
+    for(i = k; i < n - 1; i++)
+        for(j = i + 1; j < n; j++)
+            if(arr1[i] > arr[j])
+                temp = arr1[i]
+                arr1[i] = arr1[j]
+                arr1[j] = temp
+                temp = arr[i] % 10
+                arr[i] = arr[j] % 10
+                arr[j] = temp
+            
+```
+
+## Complexity 
+
+    Time Complexity =  d * (n + k) 
+    Space Complexity =   n + 2^d
+
+where
+>d = number of digits<br/>
+>n = number of keys<br/>
+>k = size of bucket
+      
+## Implementation
+* [C# Code](https://github.com/jainaman224/Algo_Ds_Notes/blob/master/Postman_Sort/Postman_Sort.cs)
+