QueueReconstructionByHeight.java

package com.freetymekiyan.algorithms.level.medium;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;

/**
 * Suppose you have a random list of people standing in a queue. Each person is described by a pair of integers (h, k),
 * where h is the height of the person and k is the number of people in front of this person who have a height greater
 * than or equal to h. Write an algorithm to reconstruct the queue.
 * <p>
 * Note:
 * The number of people is less than 1,100.
 * <p>
 * Example
 * <p>
 * Input:
 * [[7,0], [4,4], [7,1], [5,0], [6,1], [5,2]]
 * <p>
 * Output:
 * [[5,0], [7,0], [5,2], [6,1], [4,4], [7,1]]
 * <p>
 * Tags: Greedy
 * Similar Problems: (H) Count of Smaller Numbers After Self
 */
public class QueueReconstructionByHeight {

    /**
     * Sort the array by height descending and k ascending.
     * Pick the highest people from the array.
     * Then pick the second highest from the array and insert to result according to k.
     * Repeat till there is no more people.
     * <p>
     * If k means the # of people in front of him that are shorter or equal.
     * We should sort by height ascending and k asceding instead.
     */
    public int[][] reconstructQueue(int[][] people) {
        Arrays.sort(people, new Comparator<int[]>() {
            @Override
            public int compare(int[] o1, int[] o2) {
                return o2[0] - o1[0] == 0 ? o1[1] - o2[1] : o2[0] - o1[0];
            }
        });
        List<int[]> res = new ArrayList<>();
        for (int[] p : people) {
            res.add(p[1], p);
        }

        return res.toArray(new int[0][0]);
    }

}