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leetcode_java/src/main/java/LeetCodeJava/DynamicProgramming/ClimbingStairs.java
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| package LeetCodeJava.DynamicProgramming; | ||
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| // https://leetcode.com/problems/climbing-stairs/ | ||
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| import java.util.*; | ||
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| public class ClimbingStairs { | ||
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| // V0 | ||
| // IDEA : DP | ||
| public int climbStairs(int n) { | ||
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| // null check | ||
| if (n <= 0){ | ||
| return 0; | ||
| } | ||
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| if (n == 1 || n == 2){ | ||
| return n; | ||
| } | ||
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| // NOTE !!! init dp as ArrayList | ||
| List<Integer> dp = new ArrayList<>(); | ||
| dp.add(1); // i = 0 | ||
| dp.add(1); // i = 1 | ||
| //System.out.println("dp = " + dp); | ||
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| // NOTE !!! start from i = 2 | ||
| for (int i = 2; i <= n; i++){ | ||
| int tmp = dp.get(i-1) + dp.get(i-2); | ||
| dp.add(tmp); | ||
| } | ||
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| return dp.get(n); | ||
| } | ||
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| // V1 | ||
| // IDEA : Memoization | ||
| // https://leetcode.com/problems/climbing-stairs/solutions/3708750/4-method-s-beat-s-100-c-java-python-beginner-friendly/ | ||
| public int climbStairs_2(int n) { | ||
| Map<Integer, Integer> memo = new HashMap<>(); | ||
| return climbStairs(n, memo); | ||
| } | ||
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| private int climbStairs(int n, Map<Integer, Integer> memo) { | ||
| if (n == 0 || n == 1) { | ||
| return 1; | ||
| } | ||
| if (!memo.containsKey(n)) { | ||
| memo.put(n, climbStairs(n-1, memo) + climbStairs(n-2, memo)); | ||
| } | ||
| return memo.get(n); | ||
| } | ||
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| // V2 | ||
| // IDEA : Tabulation | ||
| // https://leetcode.com/problems/climbing-stairs/solutions/3708750/4-method-s-beat-s-100-c-java-python-beginner-friendly/ | ||
| public int climbStairs_4(int n) { | ||
| if (n == 0 || n == 1) { | ||
| return 1; | ||
| } | ||
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| int[] dp = new int[n+1]; | ||
| dp[0] = dp[1] = 1; | ||
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| for (int i = 2; i <= n; i++) { | ||
| dp[i] = dp[i-1] + dp[i-2]; | ||
| } | ||
| return dp[n]; | ||
| } | ||
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| // V3 | ||
| // IDEA : Space Optimization | ||
| // https://leetcode.com/problems/climbing-stairs/solutions/3708750/4-method-s-beat-s-100-c-java-python-beginner-friendly/ | ||
| public int climbStairs_5(int n) { | ||
| if (n == 0 || n == 1) { | ||
| return 1; | ||
| } | ||
| int prev = 1, curr = 1; | ||
| for (int i = 2; i <= n; i++) { | ||
| int temp = curr; | ||
| curr = prev + curr; | ||
| prev = temp; | ||
| } | ||
| return curr; | ||
| } | ||
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| } |