Create 1137. N-th Tribonacci Number.java

Author: cyzlcyzl

leetcode3/염혜정/1137. N-th Tribonacci Number.java

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+// O(3n)
+
+class Solution {
+    public int tribonacci(int n) {
+        if (n <= 1) return n;
+
+        int[] num = new int[n+1];
+        num[1] = 1; num[2] = 1;
+        if (n == 2) return num[2];
+
+        for (int i = 3; i<=n; i++) {
+            for (int k = i-3; k<i; k++) num[i] += num[k];
+        }
+        return num[n];
+    }
+}