LeetCode/java/find-building-where-alice-and-bob-can-meet.java

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

class Solution {
  private record Query(int height, int index) {}

  private static List<List<Query>> process(int[] heights, int[][] originalQueries, int[] result) {
    List<List<Query>> queries = new ArrayList<>(heights.length);
    for (var i = 0; i < heights.length; ++i) {
      queries.add(new ArrayList<>());
    }

    for (var i = 0; i < originalQueries.length; ++i) {
      int a = Math.min(originalQueries[i][0], originalQueries[i][1]);
      int b = Math.max(originalQueries[i][0], originalQueries[i][1]);

      if (heights[b] > heights[a] || a == b) {
        result[i] = b;
      } else {
        queries.get(b).add(new Query(heights[a], i));
      }
    }

    return queries;
  }

  private int search(List<Query> stack, int height) {
    int l = 0;
    int r = stack.size() - 1;

    var index = -1;
    while (l <= r) {
      var mid = (l + r) / 2;
      if (stack.get(mid).height > height) {
        index = Math.max(index, mid);
        l = mid + 1;
      } else {
        r = mid - 1;
      }
    }

    return index;
  }

  public int[] leftmostBuildingQueries(int[] heights, int[][] originalQueries) {
    var result = new int[originalQueries.length];
    Arrays.fill(result, -1);

    var queries = process(heights, originalQueries, result);

    List<Query> stack = new ArrayList<>();
    for (var i = heights.length - 1; i >= 0; --i) {
      var size = stack.size();
      for (var q : queries.get(i)) {
        var pos = search(stack, q.height);
        if (pos < size && pos >= 0) {
          result[q.index] = stack.get(pos).index;
        }
      }

      while (!stack.isEmpty() && stack.get(stack.size() - 1).height <= heights[i]) {
        stack.remove(stack.size() - 1);
      }

      stack.add(new Query(heights[i], i));
    }

    return result;
  }
}