leetcode 0801

rust-leetcode/README.md

@@ -85,4 +85,5 @@ Leetcode Rust 实现
 * [面试题10- I 斐波那契数列](src/interview_10_01.rs)
 * [633 平方数之和](src/leetcode_633.rs)
 * [204 计数质数](src/leetcode_204.rs)
-* [686 重复叠加字符串匹配](src/leetcode_686.rs)
\ No newline at end of file
+* [686 重复叠加字符串匹配](src/leetcode_686.rs)
+* [面试题 08.01 三步问题](src/interview_08_01.rs)
\ No newline at end of file

rust-leetcode/src/interview_08_01.rs

+use crate::pre_structs::Solution;
+
+/// 面试题 08.01. 三步问题
+/// 三步问题。有个小孩正在上楼梯，楼梯有n阶台阶，小孩一次可以上1阶、2阶或3阶。实现一种方法，计算小孩有多少种上楼梯的方式。结果可能很大，你需要对结果模1000000007。
+impl Solution {
+    //对于最后一步有三种可能，相加即可F(n-1)+F(n-2)+F(n-3)
+    //递归 n=61超时
+    pub fn ways_to_step(n: i32) -> i32 {
+        if n == 1 {
+            return 1;
+        }
+        if n == 2 {
+            return 2;
+        }
+        if n == 3 {
+            return 4;
+        }
+        return {
+            let mut ret = Solution::ways_to_step(n - 1)
+                + Solution::ways_to_step(n - 2)
+                + Solution::ways_to_step(n - 3);
+            if ret > 1000000007 {
+                ret -= 1000000007;
+            } else {
+                ret;
+            }
+            ret
+        };
+    }
+
+    //8MS  80%,9.1MB 100%
+    pub fn ways_to_step_(n: i32) -> i32 {
+        if n == 1 {
+            return 1;
+        }
+        if n == 2 {
+            return 2;
+        }
+        if n == 3 {
+            return 4;
+        }
+        let mut dp = vec![0i64; (n + 1) as usize];
+        dp[1] = 1;
+        dp[2] = 2;
+        dp[3] = 4;
+        for i in 4usize..=n as usize {
+            let mut t = dp[i - 1] + dp[i - 2] + dp[i - 3];
+            if t > 1000000007 {
+                t %= 1000000007;
+            } else {
+                t;
+            }
+            dp[i] = t;
+        }
+        (dp[n as usize]) as i32
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use crate::pre_structs::Solution;
+
+    #[test]
+    fn ways_to_step() {
+        let ret = Solution::ways_to_step(5);
+        println!("{}", ret);
+        let ret2 = Solution::ways_to_step_(61);
+        println!("{}", ret2);
+        let ret3 = Solution::ways_to_step_(5);
+        assert_eq!(ret, 13);
+        assert_eq!(ret2, 752119970);
+        assert_eq!(ret3, 13);
+    }
+}

rust-leetcode/src/main.rs

@@ -6,6 +6,7 @@ pub mod interview_03_04;
 pub mod interview_04_02;
 pub mod interview_05;
 pub mod interview_06;
+pub mod interview_08_01;
 pub mod interview_09;
 pub mod interview_10_01;
 pub mod interview_16_07;