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CS1101S M16 d_merge_sort.js
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CS1101S M16 d_merge_sort.js
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// Q1
function d_split_list(xs) {
const len = length(xs);
function helper(lst,i) {
return i === 0
? lst
: helper(tail(lst),i-1);
}
const lasthalf = helper(xs,math_ceil(len/2));
let ys = xs;
for (let i = 0; i<(len/2)-1;i=i+1) {
ys = tail(ys);
}
set_tail(ys,null);
return pair(xs,lasthalf);
}
// TEST:
const my_list1 = list(1, 2, 3, 4, 5, 6);
const my_list2 = list(5, 4, 3, 2, 1);
d_split_list(my_list1);
d_split_list(my_list2);
// Q2
function d_merge(xs, ys) {
function swaptailofxsswithyss(xss,yss) {
if (is_null(xss) || is_null(tail(xss))) {
set_tail(xss,yss);
return xs;
}
else if (head(tail(xss))<head(yss)) {
return swaptailofxsswithyss(tail(xss),yss);
} else {
let temp = null;
temp = tail(xss);
set_tail(xss,yss);
yss = temp;
if (is_null(yss)) {
return xs;
} else {
return swaptailofxsswithyss(tail(xss),yss);
}
}
}
if (head(xs)>head(ys)) {
return d_merge(ys,xs);
}
return swaptailofxsswithyss(xs,ys);
}
// TEST:
// const my_list1 = list(2, 4, 5, 9);
// const my_list2 = list(3, 5, 8);
const my_list1 = list(1, 4, 5);
const my_list2 = list(2, 3, 9);
d_merge(my_list1, my_list2);
// Q3
function d_merge_sort(xs) {
if (is_null(xs) || is_null(tail(xs))) {
return xs;
} else {
const splited = d_split_list(xs);
const first_half = head(splited);
const last_half = tail(splited);
return d_merge(d_merge_sort(first_half),d_merge_sort(last_half));
}
}