Skip to content

Files

Latest commit

 

History

History
292 lines (250 loc) · 6.84 KB

README_EN.md

File metadata and controls

292 lines (250 loc) · 6.84 KB

91. Decode Ways

中文文档

Description

A message containing letters from A-Z can be encoded into numbers using the following mapping:

'A' -> "1"
'B' -> "2"
...
'Z' -> "26"

To decode an encoded message, all the digits must be grouped then mapped back into letters using the reverse of the mapping above (there may be multiple ways). For example, "11106" can be mapped into:

  • "AAJF" with the grouping (1 1 10 6)
  • "KJF" with the grouping (11 10 6)

Note that the grouping (1 11 06) is invalid because "06" cannot be mapped into 'F' since "6" is different from "06".

Given a string s containing only digits, return the number of ways to decode it.

The test cases are generated so that the answer fits in a 32-bit integer.

 

Example 1:

Input: s = "12"
Output: 2
Explanation: "12" could be decoded as "AB" (1 2) or "L" (12).

Example 2:

Input: s = "226"
Output: 3
Explanation: "226" could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).

Example 3:

Input: s = "06"
Output: 0
Explanation: "06" cannot be mapped to "F" because of the leading zero ("6" is different from "06").

 

Constraints:

  • 1 <= s.length <= 100
  • s contains only digits and may contain leading zero(s).

Solutions

Dynamic programming.

Python3

class Solution:
    def numDecodings(self, s: str) -> int:
        n = len(s)
        f = [1] + [0] * n
        for i, c in enumerate(s, 1):
            if c != "0":
                f[i] = f[i - 1]
            if i > 1 and s[i - 2] != "0" and int(s[i - 2 : i]) <= 26:
                f[i] += f[i - 2]
        return f[n]
class Solution:
    def numDecodings(self, s: str) -> int:
        f, g = 0, 1
        for i, c in enumerate(s, 1):
            h = g if c != "0" else 0
            if i > 1 and s[i - 2] != "0" and int(s[i - 2 : i]) <= 26:
                h += f
            f, g = g, h
        return g

Java

class Solution {
    public int numDecodings(String s) {
        int n = s.length();
        int[] f = new int[n + 1];
        f[0] = 1;
        for (int i = 1; i <= n; ++i) {
            if (s.charAt(i - 1) != '0') {
                f[i] = f[i - 1];
            }
            if (i > 1 && s.charAt(i - 2) != '0' && Integer.valueOf(s.substring(i - 2, i)) <= 26) {
                f[i] += f[i - 2];
            }
        }
        return f[n];
    }
}
class Solution {
    public int numDecodings(String s) {
        int n = s.length();
        int f = 0, g = 1;
        for (int i = 1; i <= n; ++i) {
            int h = s.charAt(i - 1) != '0' ? g : 0;
            if (i > 1 && s.charAt(i - 2) != '0' && Integer.valueOf(s.substring(i - 2, i)) <= 26) {
                h += f;
            }
            f = g;
            g = h;
        }
        return g;
    }
}

C++

class Solution {
public:
    int numDecodings(string s) {
        int n = s.size();
        int f[n + 1];
        memset(f, 0, sizeof(f));
        f[0] = 1;
        for (int i = 1; i <= n; ++i) {
            if (s[i - 1] != '0') {
                f[i] = f[i - 1];
            }
            if (i > 1 && (s[i - 2] == '1' || s[i - 2] == '2' && s[i - 1] <= '6')) {
                f[i] += f[i - 2];
            }
        }
        return f[n];
    }
};
class Solution {
public:
    int numDecodings(string s) {
        int n = s.size();
        int f = 0, g = 1;
        for (int i = 1; i <= n; ++i) {
            int h = s[i - 1] != '0' ? g : 0;
            if (i > 1 && (s[i - 2] == '1' || (s[i - 2] == '2' && s[i - 1] <= '6'))) {
                h += f;
            }
            f = g;
            g = h;
        }
        return g;
    }
};

Go

func numDecodings(s string) int {
	n := len(s)
	f := make([]int, n+1)
	f[0] = 1
	for i := 1; i <= n; i++ {
		if s[i-1] != '0' {
			f[i] = f[i-1]
		}
		if i > 1 && (s[i-2] == '1' || (s[i-2] == '2' && s[i-1] <= '6')) {
			f[i] += f[i-2]
		}
	}
	return f[n]
}
func numDecodings(s string) int {
	n := len(s)
	f, g := 0, 1
	for i := 1; i <= n; i++ {
		h := 0
		if s[i-1] != '0' {
			h = g
		}
		if i > 1 && (s[i-2] == '1' || (s[i-2] == '2' && s[i-1] <= '6')) {
			h += f
		}
		f, g = g, h
	}
	return g
}

TypeScript

function numDecodings(s: string): number {
    const n = s.length;
    const f: number[] = new Array(n + 1).fill(0);
    f[0] = 1;
    for (let i = 1; i <= n; ++i) {
        if (s[i - 1] !== '0') {
            f[i] = f[i - 1];
        }
        if (i > 1 && (s[i - 2] === '1' || (s[i - 2] === '2' && s[i - 1] <= '6'))) {
            f[i] += f[i - 2];
        }
    }
    return f[n];
}
function numDecodings(s: string): number {
    const n = s.length;
    let [f, g] = [0, 1];
    for (let i = 1; i <= n; ++i) {
        let h = s[i - 1] !== '0' ? g : 0;
        if (i > 1 && (s[i - 2] === '1' || (s[i - 2] === '2' && s[i - 1] <= '6'))) {
            h += f;
        }
        [f, g] = [g, h];
    }
    return g;
}

C#

public class Solution {
    public int NumDecodings(string s) {
        int n = s.Length;
        int[] f = new int[n + 1];
        f[0] = 1;
        for (int i = 1; i <= n; ++i) {
            if (s[i - 1] != '0') {
                f[i] = f[i - 1];
            }
            if (i > 1 && (s[i - 2] == '1' || (s[i - 2] == '2' && s[i - 1] <= '6'))) {
                f[i] += f[i - 2];
            }
        }
        return f[n];
    }
}
public class Solution {
    public int NumDecodings(string s) {
        int n = s.Length;
        int f = 0, g = 1;
        for (int i = 1; i <= n; ++i) {
            int h = s[i - 1] != '0' ? g : 0;
            if (i > 1 && (s[i - 2] == '1' || (s[i - 2] == '2' && s[i - 1] <= '6'))) {
                h += f;
            }
            f = g;
            g = h;
        }
        return g;
    }
}

...