67 lines
1.5 KiB
Java
67 lines
1.5 KiB
Java
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import java.util.*;
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public class MatrixChainMultiply {
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public static int solveIterative(int[] s) {
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int n = s.length - 1;
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int[][] p = new int[n][n];
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int[][] m = new int[n][n];
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for (int len = 2; len <= n; len++) {
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for (int a = 0; a + len <= n; a++) {
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int b = a + len - 1;
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m[a][b] = Integer.MAX_VALUE;
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for (int c = a; c < b; c++) {
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int v = m[a][c] + m[c + 1][b] + s[a] * s[c + 1] * s[b + 1];
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if (m[a][b] > v) {
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m[a][b] = v;
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p[a][b] = c;
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}
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}
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}
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}
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return m[0][n - 1];
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}
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public static int solveRecursive(int[] s) {
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int n = s.length - 1;
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int[][] cache = new int[n][n];
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for (int[] x : cache)
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Arrays.fill(x, INF);
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int[][] p = new int[n][n];
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return rec(0, n - 1, s, p, cache);
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}
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static final int INF = Integer.MAX_VALUE / 3;
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static int rec(int i, int j, int[] s, int[][] p, int[][] cache) {
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if (i == j)
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return 0;
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int res = cache[i][j];
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if (res != INF)
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return res;
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for (int k = i; k < j; k++) {
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int v = rec(i, k, s, p, cache) + rec(k + 1, j, s, p, cache) + s[i] * s[k + 1] * s[j + 1];
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if (res > v) {
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res = v;
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p[i][j] = k;
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}
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}
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return cache[i][j] = res;
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}
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// test
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public static void main(String[] args) {
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Random rnd = new Random(1);
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for (int step = 0; step < 1000; step++) {
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int n = rnd.nextInt(6) + 2;
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int[] s = rnd.ints(n, 1, 11).toArray();
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int res1 = solveIterative(s);
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int res2 = solveRecursive(s);
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if (res1 != res2) {
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System.out.println(res1 + " " + res2);
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System.out.println(Arrays.toString(s));
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}
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}
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}
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}
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