557. Reverse Words in a String III

Stack

class Solution {
    public String reverseWords(String s) {
        StringBuilder sb = new StringBuilder();

        Stack<Character> stack = new Stack<>();
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (c == ' ') {
                while (!stack.isEmpty()) {
                    sb.append(stack.pop());
                }
                sb.append(c);
            } else {
                stack.push(c);
            }
        }

        while (!stack.isEmpty()) {
            sb.append(stack.pop());
        }

        return sb.toString();
    }
}

Two Pointers

class Solution {
    public String reverseWords(String s) {
        StringBuilder sb = new StringBuilder();

        int i = 0;
        while (i < s.length()) {
            int startIndex = i;
            while (i < s.length() && s.charAt(i) != ' ') {
                i++;
            }
            // 跳出时 i 位于字符串末端或者 i 停留在空格上

            for (int k = i - 1; k >= startIndex; k--) {
                sb.append(s.charAt(k));
            }

            while (i < s.length() && s.charAt(i) == ' ') {
                sb.append(' ');
                i++;
            }
        }

        return sb.toString();
    }
}

Two Pointers

class Solution {
    public String reverseWords(String s) {
        char[] charArray = s.toCharArray();

        int i = 0;
        while (i < charArray.length) {
            int startIndex = i;
            while (i < charArray.length && charArray[i] != ' ') {
                i++;
            }

            int left = startIndex, right = i - 1;
            while (left < right) {
                swap(charArray, left++, right--);
            }

            while (i < charArray.length && charArray[i] == ' ') {
                i++;
            }
        }

        return new String(charArray);
    }

    private void swap(char[] chars, int i, int j) {
        char tmp = chars[i];
        chars[i] = chars[j];
        chars[j] = tmp;
    }
}

Reference

557. Reverse Words in a String III