Реализации алгоритмов/Венгерский алгоритм

Венгерский алгоритм — алгоритм оптимизации, решающий задачу о назначениях за полиномиальное время.

Java[править]

Реализация Венгерского алгоритма с временной сложность O(n³) для задачи о назначениях (минимизация стоимости). Предполагается, что матрица является квадратной. Код для проверки этого может быть легко добавлен в конструктор. Вызов code>new Hungarian(costMatrix).execute() возвращает двумерный массив, где result[i][0] и result[i][1] — индексы строки и столбца назначения i. Алгоритм работает за время O(n³) (или, по крайней мере, так должно быть) и использует O(n²) памяти, улучшаемо^[1]:

import java.util.ArrayList;

public class Hungarian {

	private int numRows;
	private int numCols;

	private boolean[][] primes;
	private boolean[][] stars;
	private boolean[] rowsCovered;
	private boolean[] colsCovered;
	private float[][] costs;

	public Hungarian(float theCosts[][]) {
		costs = theCosts;
		numRows = costs.length;
		numCols = costs[0].length;

		primes = new boolean[numRows][numCols];
		stars = new boolean[numRows][numCols];

		// Инициализация массивов с покрытием строк/столбцов
		rowsCovered = new boolean[numRows];
		colsCovered = new boolean[numCols];
		for (int i = 0; i < numRows; i++) {
			rowsCovered[i] = false;
		}
		for (int j = 0; j < numCols; j++) {
			colsCovered[j] = false;
		}
		// Инициализация матриц
		for (int i = 0; i < numRows; i++) {
			for (int j = 0; j < numCols; j++) {
				primes[i][j] = false;
				stars[i][j] = false;
			}
		}
	}

	public int[][] execute() {
		subtractRowColMins();

		this.findStars(); // O(n^2)
		this.resetCovered(); // O(n);
		this.coverStarredZeroCols(); // O(n^2)

		while (!allColsCovered()) {
			int[] primedLocation = this.primeUncoveredZero(); // O(n^2)

			// It's possible that we couldn't find a zero to prime, so we have to induce some zeros so we can find one to prime
			if (primedLocation[0] == -1) {
				this.minUncoveredRowsCols(); // O(n^2)
				primedLocation = this.primeUncoveredZero(); // O(n^2)
			}

			// is there a starred 0 in the primed zeros row?
			int primedRow = primedLocation[0];
			int starCol = this.findStarColInRow(primedRow);
			if (starCol != -1) {
				// cover ther row of the primedLocation and uncover the star column
				rowsCovered[primedRow] = true;
				colsCovered[starCol] = false;
			} else { // otherwise we need to find an augmenting path and start over.
				this.augmentPathStartingAtPrime(primedLocation);
				this.resetCovered();
				this.resetPrimes();
				this.coverStarredZeroCols();
			}
		}

		return this.starsToAssignments(); // O(n^2)

	}

	/*
	 * the starred 0's in each column are the assignments.
	 * O(n^2)
	 */
	public int[][] starsToAssignments() {
		int[][] toRet = new int[numCols][];
		for (int j = 0; j < numCols; j++) {
			toRet[j] = new int[] {
				this.findStarRowInCol(j), j
			}; // O(n)
		}
		return toRet;
	}

	/*
	 * resets prime information
	 */
	public void resetPrimes() {
		for (int i = 0; i < numRows; i++) {
			for (int j = 0; j < numCols; j++) {
				primes[i][j] = false;
			}
		}
	}


	/*
	 * resets covered information, O(n)
	 */
	public void resetCovered() {
		for (int i = 0; i < numRows; i++) {
			rowsCovered[i] = false;
		}
		for (int j = 0; j < numCols; j++) {
			colsCovered[j] = false;
		}
	}

	/*
	 * get the first zero in each column, star it if there isn't already a star in that row
	 * cover the row and column of the star made, and continue to the next column
	 * O(n^2)
	 */
	public void findStars() {
		boolean[] rowStars = new boolean[numRows];
		boolean[] colStars = new boolean[numCols];

		for (int i = 0; i < numRows; i++) {
			rowStars[i] = false;
		}
		for (int j = 0; j < numCols; j++) {
			colStars[j] = false;
		}

		for (int j = 0; j < numCols; j++) {
			for (int i = 0; i < numRows; i++) {
				if (costs[i][j] == 0 && !rowStars[i] && !colStars[j]) {
					stars[i][j] = true;
					rowStars[i] = true;
					colStars[j] = true;
					break;
				}
			}
		}
	}

	/*
	 * Finds the minimum uncovered value, and adds it to all the covered rows then
	 * subtracts it from all the uncovered columns. This results in a cost matrix with
	 * at least one more zero.
	 */
	private void minUncoveredRowsCols() {
		// find min uncovered value
		float minUncovered = Float.MAX_VALUE;
		for (int i = 0; i < numRows; i++) {
			if (!rowsCovered[i]) {
				for (int j = 0; j < numCols; j++) {
					if (!colsCovered[j]) {
						if (costs[i][j] < minUncovered) {
							minUncovered = costs[i][j];
						}
					}
				}
			}
		}

		// add that value to all the COVERED rows.
		for (int i = 0; i < numRows; i++) {
			if (rowsCovered[i]) {
				for (int j = 0; j < numCols; j++) {
					costs[i][j] = costs[i][j] + minUncovered;

				}
			}
		}

		// subtract that value from all the UNcovered columns
		for (int j = 0; j < numCols; j++) {
			if (!colsCovered[j]) {
				for (int i = 0; i < numRows; i++) {
					costs[i][j] = costs[i][j] - minUncovered;
				}
			}
		}
	}

	/*
	 * Finds an uncovered zero, primes it, and returns an array
	 * describing the row and column of the newly primed zero.
	 * If no uncovered zero could be found, returns -1 in the indices.
	 * O(n^2)
	 */
	private int[] primeUncoveredZero() {
		int[] location = new int[2];

		for (int i = 0; i < numRows; i++) {
			if (!rowsCovered[i]) {
				for (int j = 0; j < numCols; j++) {
					if (!colsCovered[j]) {
						if (costs[i][j] == 0) {
							primes[i][j] = true;
							location[0] = i;
							location[1] = j;
							return location;
						}
					}
				}
			}
		}

		location[0] = -1;
		location[1] = -1;
		return location;
	}

	/*
	 * Starting at a given primed location[0=row,1=col], we find an augmenting path
	 * consisting of a primed , starred , primed , ..., primed. (note that it begins and ends with a prime)
	 * We do this by starting at the location, going to a starred zero in the same column, then going to a primed zero in
	 * the same row, etc, until we get to a prime with no star in the column.
	 * O(n^2)
	 */
	private void augmentPathStartingAtPrime(int[] location) {
		// Make the arraylists sufficiently large to begin with
		ArrayList < int[] > primeLocations = new ArrayList < int[] > (numRows + numCols);
		ArrayList < int[] > starLocations = new ArrayList < int[] > (numRows + numCols);
		primeLocations.add(location);

		int currentRow = location[0];
		int currentCol = location[1];
		while (true) { // add stars and primes in pairs
			int starRow = findStarRowInCol(currentCol);
			// at some point we won't be able to find a star. if this is the case, break.
			if (starRow == -1) {
				break;
			}
			int[] starLocation = new int[] {
				starRow, currentCol
			};
			starLocations.add(starLocation);
			currentRow = starRow;

			int primeCol = findPrimeColInRow(currentRow);
			int[] primeLocation = new int[] {
				currentRow, primeCol
			};
			primeLocations.add(primeLocation);
			currentCol = primeCol;
		}

		unStarLocations(starLocations);
		starLocations(primeLocations);
	}


	/*
	 * Given an arraylist of  locations, star them
	 */
	private void starLocations(ArrayList < int[] > locations) {
		for (int k = 0; k < locations.size(); k++) {
			int[] location = locations.get(k);
			int row = location[0];
			int col = location[1];
			stars[row][col] = true;
		}
	}

	/*
	 * Given an arraylist of starred locations, unstar them
	 */
	private void unStarLocations(ArrayList < int[] > starLocations) {
		for (int k = 0; k < starLocations.size(); k++) {
			int[] starLocation = starLocations.get(k);
			int row = starLocation[0];
			int col = starLocation[1];
			stars[row][col] = false;
		}
	}


	/*
	 * Given a row index, finds a column with a prime. returns -1 if this isn't possible.
	 */
	private int findPrimeColInRow(int theRow) {
		for (int j = 0; j < numCols; j++) {
			if (primes[theRow][j]) {
				return j;
			}
		}
		return -1;
	}




	/*
	 * Given a column index, finds a row with a star. returns -1 if this isn't possible.
	 */
	public int findStarRowInCol(int theCol) {
		for (int i = 0; i < numRows; i++) {
			if (stars[i][theCol]) {
				return i;
			}
		}
		return -1;
	}


	public int findStarColInRow(int theRow) {
		for (int j = 0; j < numCols; j++) {
			if (stars[theRow][j]) {
				return j;
			}
		}
		return -1;
	}

	// looks at the colsCovered array, and returns true if all entries are true, false otherwise
	private boolean allColsCovered() {
		for (int j = 0; j < numCols; j++) {
			if (!colsCovered[j]) {
				return false;
			}
		}
		return true;
	}

	/*
	 * sets the columns covered if they contain starred zeros
	 * O(n^2)
	 */
	private void coverStarredZeroCols() {
		for (int j = 0; j < numCols; j++) {
			colsCovered[j] = false;
			for (int i = 0; i < numRows; i++) {
				if (stars[i][j]) {
					colsCovered[j] = true;
					break; // break inner loop to save a bit of time
				}
			}
		}
	}

	private void subtractRowColMins() {
		for (int i = 0; i < numRows; i++) { //for each row
			float rowMin = Float.MAX_VALUE;
			for (int j = 0; j < numCols; j++) { // grab the smallest element in that row
				if (costs[i][j] < rowMin) {
					rowMin = costs[i][j];
				}
			}
			for (int j = 0; j < numCols; j++) { // subtract that from each element
				costs[i][j] = costs[i][j] - rowMin;
			}
		}

		for (int j = 0; j < numCols; j++) { // for each col
			float colMin = Float.MAX_VALUE;
			for (int i = 0; i < numRows; i++) { // grab the smallest element in that column
				if (costs[i][j] < colMin) {
					colMin = costs[i][j];
				}
			}
			for (int i = 0; i < numRows; i++) { // subtract that from each element
				costs[i][j] = costs[i][j] - colMin;
			}
		}
	}

}

Примечания[править]

↑ Shawn O'Neil An implimentation of the O(n^3) Hungarian method for the minimum cost assignment problem

Ссылки[править]

Не все перечисленные ниже реализации удовлетворяют ограничению времени исполнения $O(n^{3})$ .

[1] Shawn O'Neil An implimentation of the O(n^3) Hungarian method for the minimum cost assignment problem

[1]