Given an input string , reverse the string word by word. 

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Input:  ["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"]

Reverse the whole string, then reverse each word.

Use start to mark the start of each string.

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public void reverseWords(char[] s) {
    reverse(s, 0, s.length - 1);

    int start = 0;
    for (int i = 0; i < s.length; ++i) {
        if (s[i] == ' ') {
            reverse(s, start, i-1);
            start = i + 1;
        }
    }
    reverse(s, start, s.length - 1);
}

public void reverse(char[] s, int start, int end) {
    while (start < end) {
        char tmp = s[start];
        s[start] = s[end];
        s[end] = tmp;
        start++;
        end--;
    }
}