Java solution 456 && 496

Author: gavinfish

java/_456132Pattern.java

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+import java.util.Stack;
+
+/**
+ * Given a sequence of n integers a1, a2, ..., an, a 132 pattern is a subsequence ai, aj, ak such that i < j < k and ai < ak < aj. Design an algorithm that takes a list of n numbers as input and checks whether there is a 132 pattern in the list.
+ * <p>
+ * Note: n will be less than 15,000.
+ * <p>
+ * Example 1:
+ * Input: [1, 2, 3, 4]
+ * Output: False
+ * Explanation: There is no 132 pattern in the sequence.
+ * <p>
+ * Example 2:
+ * Input: [3, 1, 4, 2]
+ * Output: True
+ * Explanation: There is a 132 pattern in the sequence: [1, 4, 2].
+ * <p>
+ * Example 3:
+ * Input: [-1, 3, 2, 0]
+ * Output: True
+ * Explanation: There are three 132 patterns in the sequence: [-1, 3, 2], [-1, 3, 0] and [-1, 2, 0].
+ * <p>
+ * Created by drfish on 6/13/2017.
+ */
+public class _456132Pattern {
+    public boolean find132pattern(int[] nums) {
+        int num3 = Integer.MIN_VALUE;
+        Stack<Integer> stack = new Stack<>();
+        for (int i = nums.length - 1; i >= 0; i--) {
+            if (nums[i] < num3) {
+                return true;
+                // always track the largest third num
+            } else while (!stack.isEmpty() && nums[i] > stack.peek()) {
+                num3 = stack.pop();
+            }
+            stack.push(nums[i]);
+        }
+        return false;
+    }
+
+    public static void main(String[] args) {
+        _456132Pattern solution = new _456132Pattern();
+        assert !solution.find132pattern(new int[]{1, 2, 3, 4});
+        assert solution.find132pattern(new int[]{3, 1, 4, 2});
+        assert solution.find132pattern(new int[]{-1, 3, 2, 0});
+    }
+}

java/_496NextGreaterElementI.java

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+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Stack;
+
+/**
+ * You are given two arrays (without duplicates) nums1 and nums2 where nums1’s elements are subset of nums2. Find all the next greater numbers for nums1's elements in the corresponding places of nums2.
+ * The Next Greater Number of a number x in nums1 is the first greater number to its right in nums2. If it does not exist, output -1 for this number.
+ * <p>
+ * Example 1:
+ * Input: nums1 = [4,1,2], nums2 = [1,3,4,2].
+ * Output: [-1,3,-1]
+ * Explanation:
+ * For number 4 in the first array, you cannot find the next greater number for it in the second array, so output -1.
+ * For number 1 in the first array, the next greater number for it in the second array is 3.
+ * For number 2 in the first array, there is no next greater number for it in the second array, so output -1.
+ * <p>
+ * Example 2:
+ * Input: nums1 = [2,4], nums2 = [1,2,3,4].
+ * Output: [3,-1]
+ * Explanation:
+ * For number 2 in the first array, the next greater number for it in the second array is 3.
+ * For number 4 in the first array, there is no next greater number for it in the second array, so output -1.
+ * <p>
+ * Note:
+ * All elements in nums1 and nums2 are unique.
+ * The length of both nums1 and nums2 would not exceed 1000.
+ * <p>
+ * Created by drfish on 6/14/2017.
+ */
+public class _496NextGreaterElementI {
+    public int[] nextGreaterElement(int[] findNums, int[] nums) {
+        Map<Integer, Integer> map = new HashMap<>();
+        Stack<Integer> stack = new Stack<>();
+
+        for (int num : nums) {
+            while (!stack.isEmpty() && num > stack.peek()) {
+                map.put(stack.pop(), num);
+            }
+            stack.push(num);
+        }
+
+        int[] result = new int[findNums.length];
+        for (int i = 0; i < findNums.length; i++) {
+            result[i] = map.getOrDefault(findNums[i], -1);
+        }
+        return result;
+    }
+
+    public static void main(String[] args) {
+        _496NextGreaterElementI solution = new _496NextGreaterElementI();
+        assert Arrays.equals(new int[]{-1, 3, -1}, solution.nextGreaterElement(new int[]{4, 1, 2}, new int[]{1, 3, 4, 2}));
+        assert Arrays.equals(new int[]{3, -1}, solution.nextGreaterElement(new int[]{2, 4}, new int[]{1, 2, 3, 4}));
+    }
+}