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');
        }
    }
}