The running time is not that fast, since I just use string concatenate.

The method is simple, every time we scan from the beginning to find the first peak element and remove it. Loop it for K times.

```
public String removeKdigits(String num, int k) {
if(k >= num.length()) return "0";
int i = 0;
while(k > 0){
i = 0;
for(; i < num.length()-1 && num.charAt(i) <= num.charAt(i+1);i++);
num = num.substring(0, i) + (i < num.length()-1? num.substring(i+1):"");
k--;
}
//get rid of heading '0'
for(i = 0; i < num.length() && num.charAt(i) == '0';i++);
return i == num.length()? "0":num.substring(i);
}
```