The Dutch national flag problem is a famous computer science related programming problem proposed by Edsger Dijkstra. The flag of the Netherlands consists of three colors : Red, White and Blue. Given balls of these three colors arranged randomly in a line (the actual number of balls does not matter), the task is to arrange them such that all balls of same color are together. The cost of moving a ball (or, equivalently, examining the color of a ball) is so high that it can be done once only for each ball.

[edit] The array case

This problem can also be viewed in terms of rearranging elements of an array. Suppose each of the possible elements could be classified into exactly one of three categories (bottom, middle, and top). For example, if all elements are in 0..1, the bottom could be defined as elements in 0..0.1, the middle as 0.1..0.3 not including 0.3 and the top as 0.3 and greater. (The choice of these values illustrates that the categories need not be equal ranges). The problem is then to produce an array such that all "bottom" elements come before (have an index less than the index of) all "middle" elements, which come before all "top" elements. And to do this sorting without later moving any element after placing it in the array.

One algorithm is to have the top group grow down from the top of the array, the bottom group grow up from the bottom, and keep the middle group just above the bottom. The algorithm stores the locations just below the top group, just above the bottom, and just above the middle in three indexes. At each step, examine the element just above the middle. If it belongs to the top group, swap it with the element just below the top. If it belongs in the bottom, swap it with the element just above the bottom. If it is in the middle, leave it. Update the appropriate index. Complexity is Θ(n) moves and examinations.

Note: Using this algorithm in quicksort to partition elements, with the middle group being elements equal to the pivot, lets quicksort avoid "resorting" elements that equal the pivot.

[edit] See also

• American flag sort

[edit] External links

• Paul E. Black, Dutch national flag at the NIST Dictionary of Algorithms and Data Structures.

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