71 lines
1.8 KiB
Java
71 lines
1.8 KiB
Java
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import java.util.*;
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public class AllNearestSmallerValues {
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// https://en.wikipedia.org/wiki/All_nearest_smaller_values
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public static int[] nsv(int[] a) {
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int n = a.length;
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int[] p = new int[n];
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for (int i = 0; i < n; i++) {
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int j = i - 1;
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while (j != -1 && a[j] >= a[i]) {
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j = p[j];
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}
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p[i] = j;
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}
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return p;
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}
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public static long maxInscribedRectangle(int[] heights) {
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int n = heights.length;
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int[] rheights = new int[n];
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for (int i = 0; i < n; i++) {
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rheights[i] = heights[n - 1 - i];
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}
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int[] lnsv = nsv(heights);
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int[] rnsv = nsv(rheights);
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long res = 0;
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for (int i = 0; i < n; i++) {
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int a = lnsv[i] + 1;
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int b = n - 1 - rnsv[n - 1 - i] - 1;
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long cur = (long) (b - a + 1) * heights[i];
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res = Math.max(res, cur);
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}
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return res;
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}
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public static long maxInscribedRectangle2(int[] heights) {
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long res = 0;
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Stack<Integer> spos = new Stack<>();
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Stack<Integer> sh = new Stack<>();
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sh.push(0);
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for (int i = 0; i <= heights.length; i++) {
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int h = i < heights.length ? heights[i] : 0;
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int pos = i;
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while (sh.peek() > h) {
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pos = spos.pop();
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res = Math.max(res, (long) sh.pop() * (i - pos));
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}
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spos.push(pos);
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sh.push(h);
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}
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return res;
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}
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// Usage example
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public static void main(String[] args) {
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System.out.println(maxInscribedRectangle(new int[]{1, 2, 3}));
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System.out.println(maxInscribedRectangle2(new int[]{1, 2, 3}));
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System.out.println(Arrays.toString(nsv(new int[]{1, 1, 3, 2})));
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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(10) + 1;
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int[] h = rnd.ints(n, 0, 10).toArray();
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long res1 = maxInscribedRectangle(h);
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long res2 = maxInscribedRectangle2(h);
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if (res1 != res2)
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throw new RuntimeException();
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}
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}
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}
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