344. Reverse String

左右交换

public void reverseString(char[] s) {
    int l = 0, r = s.length - 1;
    while (l < r) {
        char tmp = s[l];
        s[l++] = s[r];
        s[r--] = tmp;
    }
}

recursion:

public void reverseString(char[] s) {
    dfs(s, 0, s.length - 1);
}

public void dfs(char[] s, int l, int r) {
    if (l >= r) return;
    char tmp = s[l];
    s[l] = s[r];
    s[r] = tmp;
    dfs(s, l + 1, r - 1);
}