Check if two linked lists merge. If so, where

algorithmdata structureslinked list

This question may be old, but I couldn't think of an answer.

Say, there are two lists of different lengths, merging at a point; how do we know where the merging point is?

Conditions:

We don't know the length
We should parse each list only once.

Example of two merged linked lists.

Best Answer

The following is by far the greatest of all I have seen - O(N), no counters. I got it during an interview to a candidate S.N. at VisionMap.

Make an interating pointer like this: it goes forward every time till the end, and then jumps to the beginning of the opposite list, and so on. Create two of these, pointing to two heads. Advance each of the pointers by 1 every time, until they meet. This will happen after either one or two passes.

I still use this question in the interviews - but to see how long it takes someone to understand why this solution works.

Related Solutions

Javascript – How to check if an array includes a value in JavaScript

Modern browsers have Array#includes, which does exactly that and is widely supported by everyone except IE:

console.log(['joe', 'jane', 'mary'].includes('jane')); //true

You can also use Array#indexOf, which is less direct, but doesn't require polyfills for outdated browsers.

console.log(['joe', 'jane', 'mary'].indexOf('jane') >= 0); //true

Many frameworks also offer similar methods:

jQuery: $.inArray(value, array, [fromIndex])
Underscore.js: _.contains(array, value) (also aliased as _.include and _.includes)
Dojo Toolkit: dojo.indexOf(array, value, [fromIndex, findLast])
Prototype: array.indexOf(value)
MooTools: array.indexOf(value)
MochiKit: findValue(array, value)
MS Ajax: array.indexOf(value)
Ext: Ext.Array.contains(array, value)
Lodash: _.includes(array, value, [from]) (is _.contains prior 4.0.0)
Ramda: R.includes(value, array)

Notice that some frameworks implement this as a function, while others add the function to the array prototype.

Under what circumstances are linked lists useful

Linked lists are very useful when you need to do a lot of insertions and removals, but not too much searching, on a list of arbitrary (unknown at compile-time) length.

Splitting and joining (bidirectionally-linked) lists is very efficient.

You can also combine linked lists - e.g. tree structures can be implemented as "vertical" linked lists (parent/child relationships) connecting together horizontal linked lists (siblings).

Using an array based list for these purposes has severe limitations:

Adding a new item means the array must be reallocated (or you must allocate more space than you need to allow for future growth and reduce the number of reallocations)
Removing items leaves wasted space or requires a reallocation
inserting items anywhere except the end involves (possibly reallocating and) copying lots of the data up one position

Best Answer

Related Solutions

Javascript – How to check if an array includes a value in JavaScript

Under what circumstances are linked lists useful

Related Topic