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Commit cffdb17

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Merge pull request #327 from cheehwatang/add-1137-N-thTribonacciNumber
Add 1137. N-th Tribonacci Number (Recursive)
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‎README.md‎

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<th>Solution</th>
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<th>Topics</th>
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</tr>
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<tr>
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<td align="center">September 6th</td>
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<td>1137. <a href="https://leetcode.com/problems/n-th-tribonacci-number/">N-th Tribonacci Number</a></td>
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<td align="center">$\text{\color{TealBlue}Easy}$</td>
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<td align="center">
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<a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/1137.%20N-th%20Tribonacci%20Number/NthTribonacciNumber_Recursive.java">Dynamic Programming - Memoization</a>
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</td>
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<td align="center">
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<a href="#dynamic-programming">Dynamic Programming</a>,
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<a href="#math">Math</a>,
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<a href="#memoization">Memoization</a>
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</td>
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</tr>
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<tr>
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<td align="center">September 5th</td>
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<td>1137. <a href="https://leetcode.com/problems/n-th-tribonacci-number/">N-th Tribonacci Number</a></td>
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<a href="#counting">Counting</a>
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</td>
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</tr>
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<tr>
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<td align="center">September 1st</td>
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<td>16. <a href="https://leetcode.com/problems/3sum-closest/">3Sum Closest</a></td>
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<td align="center">$\text{\color{Dandelion}Medium}$</td>
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<td align="center">
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<a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/16.%203Sum%20Closest/ThreeSumClosest.java">Sorting & Two Pointers</a>
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</td>
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<td align="center">
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<a href="#array">Array</a>,
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<a href="#sorting">Sorting</a>,
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<a href="#two-pointers">Two Pointers</a>
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</td>
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</tr>
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</table>
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</br>
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<hr>
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<tr>
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<td align="center">1137</td>
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<td><a href="https://leetcode.com/problems/n-th-tribonacci-number/">N-th Tribonacci Number</a></td>
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<td align="center">
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<a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/1137.%20N-th%20Tribonacci%20Number/NthTribonacciNumber_Iterative.java">Java</a>
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<td align="center">Java with Dynamic Programming
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<a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/1137.%20N-th%20Tribonacci%20Number/NthTribonacciNumber_Iterative.java">Iteratively</a> or
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<a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/1137.%20N-th%20Tribonacci%20Number/NthTribonacciNumber_Recursive.java">Recursively (Memoization)</a>
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</td>
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<td align="center">$\text{\color{TealBlue}Easy}$</td>
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<td align="center">
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<a href="#dynamic-programming">Dynamic Programming</a>,
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<a href="#math">Math</a>
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<a href="#math">Math</a>,
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<a href="#memoization">Memoization</a>
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</td>
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<td></td>
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</tr>
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<tr>
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<td align="center">1137</td>
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<td><a href="https://leetcode.com/problems/n-th-tribonacci-number/">N-th Tribonacci Number</a></td>
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<td align="center">
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<a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/1137.%20N-th%20Tribonacci%20Number/NthTribonacciNumber_Iterative.java">Java</a>
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<td align="center">Java with Dynamic Programming
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<a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/1137.%20N-th%20Tribonacci%20Number/NthTribonacciNumber_Iterative.java">Iteratively</a> or
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<a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/1137.%20N-th%20Tribonacci%20Number/NthTribonacciNumber_Recursive.java">Recursively (Memoization)</a>
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</td>
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<td align="center">$\text{\color{TealBlue}Easy}$</td>
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<td align="center">
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<a href="#dynamic-programming">Dynamic Programming</a>,
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<a href="#math">Math</a>
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<a href="#math">Math</a>,
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<a href="#memoization">Memoization</a>
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</td>
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<td></td>
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</tr>
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</td>
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<td></td>
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</tr>
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<tr>
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<td align="center">1137</td>
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<td><a href="https://leetcode.com/problems/n-th-tribonacci-number/">N-th Tribonacci Number</a></td>
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<td align="center">
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<a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/1137.%20N-th%20Tribonacci%20Number/NthTribonacciNumber_Recursive.java">Java</a>
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</td>
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<td align="center">$\text{\color{TealBlue}Easy}$</td>
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<td align="center">
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<a href="#dynamic-programming">Dynamic Programming</a>,
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<a href="#math">Math</a>,
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<a href="#memoization">Memoization</a>
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</td>
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<td>Solution Using
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<a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/1137.%20N-th%20Tribonacci%20Number/NthTribonacciNumber_Iterative.java"><em>Dynamic Programming (Iterative)</em></a>
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</td>
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</tr>
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</table>
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<p align="right"><a href="#topics"> :arrow_up:<b>Back To Top</b></a></p>
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<hr>
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package com.cheehwatang.leetcode;
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// Time Complexity : O(n),
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// where 'n' is the input 'n'.
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// The recursive call stack has a maximum height of 'n'.
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//
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// Space Complexity : O(n),
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// where 'n' is the input 'n'.
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// We use a memo of size 'n' to store the tribonacci numbers.
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// Additionally, the recursive call stack has a maximum height of 'n'.
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public class NthTribonacciNumber_Recursive {
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// Approach:
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// We use the recursive method, using the 'memo' keep track of the calculated number to lower the time complexity.
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public int tribonacci(int n) {
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// Use an integer array as 'memo' to record the results that was calculated.
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// This is so to not repeat the same calculation over the recursive calls.
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int[] memo = new int[n + 1];
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return tribonacci(n, memo);
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}
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private int tribonacci(int n, int[] memo) {
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if (n == 0 || n == 1) return n;
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if (n == 2) return 1;
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// Record the first three element in the Fibonacci sequence.
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memo[1] = 1;
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memo[2] = 1;
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// If the ways for n is not recorded in the 'memo' yet, then calculate and record the result into the memo.
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if (memo[n] == 0) {
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memo[n] = tribonacci(n - 1, memo) + tribonacci(n - 2, memo) + tribonacci(n - 3, memo);
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}
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return memo[n];
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}
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}

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