int size = s.size();

int fast = 0; // fast travels along old string

int slow = 0; // slow points to new string

for(int fast = 0; fast < size; fast++) {

if(s[fast] != ' ') { // 巧妙！if[fast] == ' '，则对slow无任何操作。其实就是略过空格

if(slow != 0) {

s[slow++] = ' ';

}

while(fast < size && s[fast] != ' ') {

s[slow++] = s[fast++];

}

}

}

s.resize(slow);

}