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Commit 82808e5

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Merge pull request #326 from cheehwatang/add-1137-N-thTribonacciNumber
Add 1137. N-th Tribonacci Number (Iterative)
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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 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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<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_Iterative.java">Dynamic Programming</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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</td>
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</tr>
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<tr>
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<td align="center">September 4th</td>
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<td>923. <a href="https://leetcode.com/problems/3sum-with-multiplicity/">3Sum With Multiplicity</a></td>
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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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<tr>
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<td align="center">August 31st</td>
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<td>15. <a href="https://leetcode.com/problems/3sum/">3Sum</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/15.%203Sum/ThreeSum.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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</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>
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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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</td>
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<td></td>
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</tr>
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<tr>
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<td align="center">1578</td>
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<td><a href="https://leetcode.com/problems/minimum-time-to-make-rope-colorful/">Minimum Time to Make Rope Colorful</a></td>
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</td>
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<td></td>
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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>
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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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</td>
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<td></td>
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</tr>
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<td align="center">1154</td>
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<td><a href="https://leetcode.com/problems/day-of-the-year/">Day of the Year</a></td>
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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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// We traverse from 3 to 'n' to calculate the n-th tribonacci number.
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//
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// Space Complexity : O(1),
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// as the auxiliary space used is independent of the input.
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public class NthTribonacciNumber_Iterative {
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// Approach:
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// We use the iterative method, using the variable 'first', 'second' and 'third' to keep track.
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public int tribonacci(int n) {
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if (n == 0 || n == 1) return n;
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if (n == 2) return 1;
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// As we need both 'n - 1', 'n - 2' and 'n - 3' to start, record 'first', 'second' and 'third', respectively.
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// For each iteration until n, sum all three numbers and replace the numbers.
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int first = 1;
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int second = 1;
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int third = 0;
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int result = 0;
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for (int i = 3; i <= n; i++) {
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result = first + second + third;
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third = second;
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second = first;
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first = result;
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}
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return result;
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}
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}

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