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Problem solution · Java

Reverse Words in a String

Reverse Words in a String: a Java solution using direct simulation. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Direct simulation
Source
walkccc LeetCode Solutions
Length
48 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Direct simulation

For Reverse Words in a String, the implementation follows the problem’s operations directly while maintaining only the state needed for the next decision.

  1. Translate each rule into one explicit state update.
  2. Maintain the invariant after every processed item.
  3. Return the accumulated state once all relevant input has been handled.

Code notes

  • 48 lines of Java from the credited upstream file 151.java.
  • The implementation keeps its working state in language-native values and containers.
  • 8 loop blocks detected.

Complexity

Count the number and nesting of passes over the input, then include the maintained containers in the memory estimate.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from walkccc LeetCode Solutions by P.-Y. Chen (walkccc) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeReverse Words in a String · JavaJava
Use this to learn the idea, then write your own version.
class Solution {  public String reverseWords(String s) {    StringBuilder sb = new StringBuilder(s).reverse(); // Reverse the whole string.    reverseWords(sb, sb.length());                     // Reverse each word.    return cleanSpaces(sb, sb.length());               // Clean up the spaces.  }   private void reverseWords(StringBuilder sb, int n) {    int i = 0;    int j = 0;     while (i < n) {      while (i < j || i < n && sb.charAt(i) == ' ') // Skip the spaces.        ++i;      while (j < i || j < n && sb.charAt(j) != ' ') // Skip the spaces.        ++j;      reverse(sb, i, j - 1); // Reverse the word.    }  }   // Trim leading, trailing, and middle spaces  private String cleanSpaces(StringBuilder sb, int n) {    int i = 0;    int j = 0;     while (j < n) {      while (j < n && sb.charAt(j) == ' ') // Skip the spaces.        ++j;      while (j < n && sb.charAt(j) != ' ') // Keep non spaces        sb.setCharAt(i++, sb.charAt(j++));      while (j < n && sb.charAt(j) == ' ') // Skip the spaces.        ++j;      if (j < n) // Keep only one space.        sb.setCharAt(i++, ' ');    }     return sb.substring(0, i).toString();  }   private void reverse(StringBuilder sb, int l, int r) {    while (l < r) {      final char temp = sb.charAt(l);      sb.setCharAt(l++, sb.charAt(r));      sb.setCharAt(r--, temp);    }  }}

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