pseudocode for Merge sort
MERGE-SORT(A, p, r)
1 if p < r
2 then q <-- [(p + r) / 2]
3 MERGE-SORT(A, p, q)
4 MERGE-SORT(A, q + 1, r)
5 MERGE-SORT(A, p, q, r)
MERGE-SORT(A, p, q, r)
1 n1 <-- q - p + 1
2 n2 <-- r - q
3 create arrays L[1 ... n1 + 1] and R[1 ... n2 + 1]
4 for i <-- 1 to n1
5 do L[i] <-- A[p + i - 1]
6 for j <-- 1 to n2
7 do R[j] <-- A[q + j]
8 L[n1 + 1] <-- infinite
9 R[n2 + 1] <-- infinite
10 i <-- 1
11 j <-- 1
12 for k <-- p to r
13 do if L[i] <= R[j]
14 then A[k] <-- L[i]
15 i <-- i + 1
16 else A[k] <-- R[j]
17 j <-- j + 1
We try to sort two piles of cards but we avoid checking whether either pile is empty in each basic step, and we use the infinite as a sentinel card to simplify our code. So, whenever the sentinel card infinie is exposed, it cannot be the smaller card unless both piles have their sentinel card exposed. But once that happens, all the non-sentinel cards have already been placed onto the output pile. Since we know in advance that exactly r - p + 1 cards will be placed onto the output pile, we can stop once we have performed that many basic steps.
Loop Invariant:
Initialization: prior to the first iteration of the loop, we have k = p, so that subarray A[p ... k - 1] is empty. this empty subarray contains the k - p = 0 smallest elements of L and R, and since i = j = 1, both L[i] and R[j] are the smallest elements of their arrays that have not been copied back into A.
Maintenance: To see that each iteration maintains the loop invariant, let us first suppose that l[i] <= R[j]. Then L[i] is the smallest element not yet copied back into A. Because A[p ... k - 1] contains the k - p smallest elements, after line 14 copies L[i] into A[k], the subarray A[p ... k] will contain the k - p + 1 smallest elements. Incrementing k(in the for loop update) and i(in line 15) re-establishes the loop invariant for the next iteration. If instead L[i] > R[j], then lines 16-17 perform the appropriate action to maintain the loop invariant.
Termination: At termination, k = r + 1. By the loop invariant, the subarray A[p ... k - 1], which is A[p ... r], contains the k - p = r - p + 1 smallest elements of L[1 ... n1 + 1] and R[1 ... n2 + 1], in sorted order. The arrays L and R together contain n1 + n2 + 2 = r - p + 3 elements. All but the two largest elements have been copied back into A, and these two largest elements are the sentinels.
