Find the Missing Number II

Giving a string with number from 1-n in random order, but miss 1 number.Find that number.

Notice n <= 30

Example

Given n = 20, str = 19201234567891011121314151618

return 17

这题是网上借鉴别人的代码 一开始没有想到要用DFS 利用这道题顺便复习了下DFS的schema 注意flag 是用来剪枝避免重复计算的

 public class Solution {
     /**
      * @param n an integer
      * @param str a string with number from 1-n
      *            in random order and miss one number
      * @return an integer
      */
     int ans=0;
     boolean flag =false;
     public int findMissing2(int n, String str) {
         // Write your code here
         boolean[] appear = new boolean[n+1];
         dfs(0, n, str, appear);
         return ans;
     }

     private void dfs(int i, int n, String s, boolean[] appear){
         if(i>=s.length()||flag){
             if(!flag){
                 for(int k=1; k<=n;k++){
                     if(!appear[k]){
                         ans = k;
                     }
                 }
             }
             flag = true;
             return;
         }
         int sum = s.charAt(i)-'0';
         if(sum ==0){
             return;
         }
         int j = i+1;
         while(sum<=n){
             if(!appear[sum]){
                 appear[sum] = true;
                 dfs(j, n, s, appear);
                 appear[sum] = false;
             }
             if(j>=s.length()) return;
             sum = 10*sum + (s.charAt(j++)-'0');
         }
     }
 }