186. Reverse Words in a String II

Description

Given a character array s, reverse the order of the words.

A word is defined as a sequence of non-space characters. The words in s will be separated by a single space.

Your code must solve the problem in-place, i.e. without allocating extra space.

 

Example 1:

Input: s = ["t","h","e"," ","s","k","y"," ","i","s"," ","b","l","u","e"]
Output: ["b","l","u","e"," ","i","s"," ","s","k","y"," ","t","h","e"]

Example 2:

Input: s = ["a"]
Output: ["a"]

 

Constraints:

  • 1 <= s.length <= 105
  • s[i] is an English letter (uppercase or lowercase), digit, or space ' '.
  • There is at least one word in s.
  • s does not contain leading or trailing spaces.
  • All the words in s are guaranteed to be separated by a single space.

Solutions

Solution 1

Python Code
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class Solution:
    def reverseWords(self, s: List[str]) -> None:
        """
        Do not return anything, modify s in-place instead.
        """

        def reverse(s, i, j):
            while i < j:
                s[i], s[j] = s[j], s[i]
                i += 1
                j -= 1

        i, j, n = 0, 0, len(s)
        while j < n:
            if s[j] == ' ':
                reverse(s, i, j - 1)
                i = j + 1
            elif j == n - 1:
                reverse(s, i, j)
            j += 1
        reverse(s, 0, n - 1)

Java Code
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class Solution {
    public void reverseWords(char[] s) {
        int n = s.length;
        for (int i = 0, j = 0; j < n; ++j) {
            if (s[j] == ' ') {
                reverse(s, i, j - 1);
                i = j + 1;
            } else if (j == n - 1) {
                reverse(s, i, j);
            }
        }
        reverse(s, 0, n - 1);
    }

    private void reverse(char[] s, int i, int j) {
        for (; i < j; ++i, --j) {
            char t = s[i];
            s[i] = s[j];
            s[j] = t;
        }
    }
}

C++ Code
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class Solution {
public:
    void reverseWords(vector<char>& s) {
        int n = s.size();
        for (int i = 0, j = 0; j < n; ++j) {
            if (s[j] == ' ') {
                reverse(s, i, j - 1);
                i = j + 1;
            } else if (j == n - 1) {
                reverse(s, i, j);
            }
        }
        reverse(s, 0, n - 1);
    }

    void reverse(vector<char>& s, int i, int j) {
        for (; i < j; ++i, --j) {
            swap(s[i], s[j]);
        }
    }
};

Go Code
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func reverseWords(s []byte) {
	n := len(s)
	for i, j := 0, 0; j < n; j++ {
		if s[j] == ' ' {
			reverse(s, i, j-1)
			i = j + 1
		} else if j == n-1 {
			reverse(s, i, j)
		}
	}
	reverse(s, 0, n-1)
}

func reverse(s []byte, i, j int) {
	for i < j {
		s[i], s[j] = s[j], s[i]
		i++
		j--
	}
}