186. Reverse Words in a String II

abc de先整个翻转：
ed cba再逐个单词翻转：
de abc就是要求的结果。

public class Solution 
{
    public void reverseWords(char[] s) 
    {
        if(s.length <= 1) return;
        
        int l = 0;
        int r = s.length-1;
        
        reverse(s,l,r);
        l = 0;
        r = 0;
        
        for(r = 0; r < s.length;r++)
        {
            if(s[r] == ' ')
            {
                reverse(s,l,r-1);
                
                l = r+1;
            }
        }
        
        r--;
        reverse(s,l,r);
        
    }
    
    public void reverse(char[] s, int l, int r)
    {
        char c = ' ';
        while(l < r)
        {
            c = s[l];
            s[l] = s[r];
            s[r] = c;
            l++;
            r--;
            
        }
    }
    

}

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二刷。

有印象是二次翻转。

边界条件要仔细。。

Time: O(n)
Space: O(n) in-place

public class Solution {
    public void reverseWords(char[] s) {
        if (s.length <= 1) return;
        int r = 0;
        int l = 0;
        while (r < s.length) {
            while (r < s.length && s[r] != ' ') r++;
            reverse(s, l, r-1);
            l = r+1;
            r++;
        }
        reverse(s, 0, s.length - 1);
    }
    
    public void reverse(char[] s, int l, int r) {
        while (l < r) {
            char c = s[l];
            s[l++] = s[r];
            s[r--] = c;
        }
        return;
    }
}