Longest Zig-Zag Subsequence

DP/LongestZigZagSubsequence.java

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+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.Map;
+
+public class LongestZigZagSubsequence {
+
+    public static void main(String args[]) {
+
+            int h[] = new int[]{10, 22, 9, 33, 49, 50, 31, 60};
+            int hs[] = h.clone();
+            Arrays.sort(hs);
+            Map<Integer, Integer> hmap = new HashMap<>();
+            for (int i = 0, j = 0; i < h.length; j++) {
+                hmap.put(hs[i], j);
+                do {
+                    ++i;
+                } while (i < h.length && hs[i] == hs[i - 1]);
+            }
+
+            for (int i = 0; i < h.length; i++)
+                h[i] = hmap.get(h[i]);
+            int m = hmap.size();
+            int f1[] = new int[m];
+            int f2[] = new int[m];
+            int f3[] = new int[m];
+
+            int ans = 0;
+            for (int i = 0; i < h.length; i++) {
+
+                int v = h[i];
+                int nf1 = Math.max(getMax(f2, m - v - 1), f3[v]) + 1;
+                int nf2 = Math.max(getMax(f1, v), f3[v]) + 1;
+
+                updateMax(f1, v, nf1);
+                updateMax(f2, m - v - 1, nf2);
+
+                ans = Math.max(ans, Math.max(nf1, nf2));
+            }
+            System.out.println(ans);
+        }
+
+        static int getMax(int fenwick[], int i) {
+            int res = fenwick[i];
+            while ((i = (i & (i + 1)) - 1) >= 0)
+                res = Math.max(res, fenwick[i]);
+            return res;
+        }
+
+        static void updateMax(int fenwick[], int i, int v) {
+            fenwick[i] = Math.max(fenwick[i], v);
+            while ((i |= i + 1) < fenwick.length)
+                fenwick[i] = Math.max(fenwick[i], v);
+        }
+    }