java: add «2940. Find Building Where Alice and Bob Can Meet»

URL:	https://leetcode.com/problems/find-building-where-alice-and-bob-can-meet/
Signed-off-by: Matej Focko <me@mfocko.xyz>

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

@@ -0,0 +1,71 @@
+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;
+  }
+}