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climbing.java
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47 lines (40 loc) · 1.17 KB
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class climbing {
public static int climbStairs(int n) {
if (n == 0 || n == 1) return 1;
// else return climbStairs(n - 1) + climbStairs(n - 2); // this is slow
// int[] nums = new int[n + 1];
// nums[0] = 1;
// nums[1] = 1;
// for (int i = 2; i <= n; i++) {
// nums[i] = nums[i-1] + nums[i-2];
// }
// return nums[n];
int n_first = 1;
int n_sec = 1;
int sum_i = 0;
for (int i = 2; i <= n; i++) {
System.out.println("i: " + i);
System.out.println("n_first: " + n_first);
System.out.println("n_sec: " + n_sec);
sum_i = n_first + n_sec; // 2
System.out.println("sum_i: " + sum_i);
n_first = n_sec; // 1
n_sec = sum_i; // 2
}
return sum_i;
}
// but we only need to save 2 numbers in the array
public static void main(String[] args) {
int n = 5;
System.out.println(climbStairs(n));
}
}
/*
* num_ways(n) = num_ways(n-1) + num_ways(n-2)
* n=0, nw = 1
* n=1, nw = 1
* n=2, nw = 2
* n=3, nw = 2+1=3
* n=4, nw=3+2=5
* count from top stair = 0
*/