Added solution to find path for robot in grid

YogenRaii · YogenRaii · commit fcb64b74053d · 2019-06-09T11:14:36.000-05:00
diff --git a/src/main/java/com/eprogrammerz/examples/algorithm/crackingCoding/MagicIndex.java b/src/main/java/com/eprogrammerz/examples/algorithm/crackingCoding/MagicIndex.java
@@ -0,0 +1,39 @@
+package com.eprogrammerz.examples.algorithm.crackingCoding;
+
+import org.junit.Test;
+
+import java.util.Arrays;
+import java.util.List;
+
+import static org.junit.Assert.assertEquals;
+
+/**
+ * A magic index in an array A[ 0••• n -1] is defined to be an index such that A[ i] = i.
+ * Given a sorted array of distinct integers, write a method to find a magic index, if one exists, in array A.
+ */
+public class MagicIndex {
+    public int findMagicIndex(List<Integer> list) {
+        return findMagicIndex(list, 0, list.size() - 1);
+    }
+
+    private int findMagicIndex(List<Integer> list, int start, int end) {
+        if (start > end) return Integer.MIN_VALUE;
+
+        int mid = start + (end - start) / 2;
+        int elem = list.get(mid);
+        if (elem == mid) return mid;
+
+        if (elem > mid) {
+            return findMagicIndex(list, start, mid - 1);
+        } else {
+            return findMagicIndex(list, mid + 1, end);
+        }
+    }
+
+    @Test
+    public void testFindMagicIndex() {
+        assertEquals(2, findMagicIndex(Arrays.asList(-1, 0, 2, 4, 5, 10, 11)));
+        assertEquals(1, findMagicIndex(Arrays.asList(-1, 1, 3, 4, 5, 6)));
+        assertEquals(Integer.MIN_VALUE, findMagicIndex(Arrays.asList(1, 2, 7, 9, 10, 12)));
+    }
+}
diff --git a/src/main/java/com/eprogrammerz/examples/algorithm/crackingCoding/RobotInGrid.java b/src/main/java/com/eprogrammerz/examples/algorithm/crackingCoding/RobotInGrid.java
@@ -0,0 +1,80 @@
+package com.eprogrammerz.examples.algorithm.crackingCoding;
+
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import java.util.Objects;
+
+import static org.hamcrest.CoreMatchers.is;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertThat;
+
+/**
+ * Robot in a Grid: Imagine a robot sitting on the upper left corner of grid with r rows and c columns.
+ * The robot can only move in two directions, right and down, but certain cells are "off limits" such that the robot cannot step on them.
+ * Design an algorithm to find a path for the robot from the top left to the bottom right.
+ */
+public class RobotInGrid {
+    public List<Pair> findPath(int[][] grid) {
+        List<Pair> path = new ArrayList<>();
+        if (grid == null) return path;
+
+        findPath(grid, 0, 0, path);
+        return path;
+    }
+
+    private boolean findPath(int[][] grid, int row, int col, List<Pair> path) {
+        if (row >= grid.length || col >= grid[0].length || grid[row][col] == 1) {
+            return false;
+        }
+
+        boolean isAtEnd = row == grid.length - 1 && col == grid[0].length - 1;
+
+        if (isAtEnd || findPath(grid, row + 1, col, path) || findPath(grid, row, col + 1, path)) {
+            path.add(0, new Pair(row, col));
+            return true;
+        }
+
+        return false;
+    }
+
+    class Pair {
+        int x;
+        int y;
+
+        Pair(int x, int y) {
+            this.x = x;
+            this.y = y;
+        }
+
+        @Override
+        public boolean equals(Object obj) {
+            if (!(obj instanceof Pair)) return false;
+
+            Pair pair = (Pair) obj;
+
+            return this.x == pair.x && this.y == pair.y;
+        }
+
+        @Override
+        public String toString() {
+            return "(" + x + ", " + y + ")";
+        }
+    }
+
+    @Test
+    public void testFindPathInGrid() {
+        int[][] grid = new int[][]{
+                {0, 0, 0},
+                {1, 1, 0},
+                {0, 0, 0}
+        };
+        List<Pair> path = findPath(grid);
+
+        List<Pair> expected = new ArrayList<>(Arrays.asList(new Pair(0, 0), new Pair(0, 1), new Pair(0, 2), new Pair(1, 2), new Pair(2, 2)));
+        assertThat(expected, is(path));
+    }
+
+}
