英文:
Map and Dynamic Programming Updating
问题
我给出的问题是:
一个孩子正在一个有n个台阶的楼梯上跑步,他可以一次跳1个台阶、2个台阶或者3个台阶。实现一个方法来计算孩子上楼梯的可能方式有多少种。
这段代码的链接是:
package main
import "fmt"
func CountWaysDP(n int, mm map[int]int) int {
if n < 0 {
return 0
} else if n == 0 {
return 1
} else if mm[n] > -1 {
return mm[n]
} else {
mm[n] = CountWaysDP(n-1, mm) +
CountWaysDP(n-2, mm) +
CountWaysDP(n-3, mm)
return mm[n]
}
}
func main() {
mm := make(map[int]int)
fmt.Println(CountWaysDP(10, mm), mm)
}
这段代码只给出了0 map[]。事实证明,动态递归在以下一行结束:
else if mm[n] > -1
那么我该如何使用动态规划来解决这个问题呢?这与《程序员面试金典》中的解决方案完全相同...
英文:
The problem I am given is
A child is running up a staircase with n steps,
and can hop either 1 step, 2 steps, or 3 steps at a time.
Implement a method to count how many possible ways the child can run up the stairs.
http://play.golang.org/p/bpjIkMm9jH
package main
import "fmt"
func CountWaysDP(n int, mm map[int]int) int {
if n < 0 {
return 0
} else if n == 0 {
return 1
} else if mm[n] > -1 {
return mm[n]
} else {
mm[n] = CountWaysDP(n-1, mm) +
CountWaysDP(n-2, mm) +
CountWaysDP(n-3, mm)
return mm[n]
}
}
func main() {
mm := make(map[int]int)
fmt.Println(CountWaysDP(10, mm), mm)
}
This just gives me 0 map[]. It turns out that the dynamic recursion ends at the following line:
else if mm[n] > -1
Then how would I use dynamic programming to solve this problem? This is exactly the same solution as in Cracking the coding interview....
答案1
得分: 2
你需要与0进行比较:
else if mm[n] > 0
当获取不存在的键的值时,map返回0。
你也可以使用数组/切片代替map,因为你知道map的键总是从1到N。
你也可以不使用递归来解决这个问题:
package main
import "fmt"
func main() {
n := 10
mm := make([]int, n+1)
mm[0] = 1
for i := 1; i <= n; i++ {
for k := 1; k <= 3; k++ {
if i-k >= 0 {
mm[i] += mm[i-k]
}
}
}
fmt.Println(mm)
fmt.Println(mm[n])
}
英文:
You need to compare with 0:
else if mm[n] > 0
map returns 0 when getting values for non existing keys.
You can also use an array/slice instead of map as you know that the map keys are always from 1 to N
You can solve this without recursion as well:
package main
import "fmt"
func main() {
n := 10
mm := make([]int, n+1)
mm[0] = 1
for i := 1; i <= n; i++ {
for k := 1; k <= 3; k++ {
if i-k >= 0 {
mm[i] += mm[i-k]
}
}
}
fmt.Println(mm)
fmt.Println(mm[n])
}
答案2
得分: 0
一个分治法的Python解决方案:
def staircase_count(nSteps):
if nSteps < 0:
return 0
if nSteps == 0:
return 1
total = 0
for step in [1, 2, 3]:
total += staircase_count(nSteps - step)
return total
assert staircase_count(1) == 1
assert staircase_count(2) == 2
assert staircase_count(3) == 4
assert staircase_count(4) == 7
英文:
A divide and conquer Python solution:
def staircase_count(nSteps):
if nSteps < 0:
return 0
if nSteps == 0:
return 1
total = 0
for step in [1, 2, 3]:
total += staircase_count(nSteps - step)
return total
assert staircase_count(1) == 1
assert staircase_count(2) == 2
assert staircase_count(3) == 4
assert staircase_count(4) == 7
答案3
得分: -1
JavaScript解决方案:(迭代)
function countPossibleWaysIterative(n) {
if (n < 0){
return -1; // 检查负数,也可以检查n是否为整数
} if (n === 0) {
return 0; // 对于0个台阶的情况
} else if (n === 1) {
return 1; // 对于1个台阶的情况
} else if (n === 2) {
return 2; // 对于2个台阶的情况
} else {
var prev_prev = 1;
var prev = 2;
var res = 4; // 对于3个台阶的情况
while (n > 3) { // 其他情况
var tmp = prev_prev + prev + res;
prev_prev = prev;
prev = res;
res = tmp;
n--;
}
}
return res;
}
英文:
JavaScript solution: ( iterative )
function countPossibleWaysIterative(n) {
if (n < 0){
return -1; // check for negative, also might want to check if n is an integer
} if (n === 0) {
return 0; // for case with 0 stairs
} else if (n === 1) {
return 1; // for case with 1 stairs
} else if (n === 2) {
return 2; // for case with 2 stairs
} else {
var prev_prev = 1;
var prev = 2;
var res = 4; // for case with 3 stairs
while (n > 3) { // all other cases
var tmp = prev_prev + prev + res;
prev_prev = prev;
prev = res;
res = tmp;
n--;
}
}
return res;
}
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