[Java][Leetcode] Array & String — Reverse Words in a String #151

SP Hou
2 min readDec 31, 2020


Given an input string 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 at least one space.

Return a string of the words in reverse order concatenated by a single space.

Note that s may contain leading or trailing spaces or multiple spaces between two words. The returned string should only have a single space separating the words. Do not include any extra spaces.

Example 1:

Input: s = "the sky is blue"
Output: "blue is sky the"

Example 2:

Input: s = "  hello world  "
Output: "world hello"
Explanation: Your reversed string should not contain leading or trailing spaces.

Example 3:

Input: s = "a good   example"
Output: "example good a"
Explanation: You need to reduce multiple spaces between two words to a single space in the reversed string.

Example 4:

Input: s = "  Bob    Loves  Alice   "
Output: "Alice Loves Bob"

Example 5:

Input: s = "Alice does not even like bob"
Output: "bob like even not does Alice"

It’s not difficult, if we solve this problem by “split”.
I try to solve it by “two-pointer”. It’s more complicated than “split”.
But, runtime and memory are better.

step1. Reverse input string (s)
step2. search space char (“ “) and keep this position (i)
step3. search next space char and keep this position (j)
step4. reverse char[] from i to j-1
step5. move char from char[] to result[] that index is finial output length