leetcode@ [241] Different Ways to Add Parentheses (Divide and Conquer)

https://leetcode.com/problems/different-ways-to-add-parentheses/

Given a string of numbers and operators, return all possible results from computing all the different possible ways to group numbers and operators. The valid operators are +, - and *.

Example 1

Input: "2-1-1".

((2-1)-1) = 0
(2-(1-1)) = 2

Output: [0, 2]

Example 2

Input: "2*3-4*5"

(2*(3-(4*5))) = -34
((2*3)-(4*5)) = -14
((2*(3-4))*5) = -10
(2*((3-4)*5)) = -10
(((2*3)-4)*5) = 10

Output: [-34, -14, -10, -10, 10]

 

为数不多的考察分治算法的题：

class Solution {
public:
    
    bool isNumber(string s) {
        for(int i=0; i<s.length(); ++i) {
            if(!(s[i] <= '9' && s[i] >= '0'))  return false;  
        }
        return true;
    }
    
    int toInt(string s) {
        if(s == "0")  return 0;
        
        int res = 0;
        for(int i=0; i<s.length(); ++i) {
            res = res*10 + (s[i]-'0');
        }
        return res;
    }
    
    vector<int> dfs(string input) {
        vector<int> load;
        if(isNumber(input)) {
            load.push_back(toInt(input));
            return load;
        }
        
        int len = input.length();
        vector<int> l, r;
        for(int i=0; i<len; ++i) {
            if(input[i] == '-') {
                l = dfs(input.substr(0, i));
                r = dfs(input.substr(i+1, len-i-1));
                for(int p=0; p<l.size(); ++p) {
                    for(int q=0; q<r.size(); ++q) {
                        load.push_back(l[p] - r[q]);
                    }
                }
            }
            else if(input[i] == '+') {
                l = dfs(input.substr(0, i));
                r = dfs(input.substr(i+1, len-i-1));
                for(int p=0; p<l.size(); ++p) {
                    for(int q=0; q<r.size(); ++q) {
                        load.push_back(l[p] + r[q]);
                    }
                }
            }
            else if(input[i] == '*') {
                l = dfs(input.substr(0, i));
                r = dfs(input.substr(i+1, len-i-1));
                for(int p=0; p<l.size(); ++p) {
                    for(int q=0; q<r.size(); ++q) {
                        load.push_back(l[p] * r[q]);
                    }
                }
            }            
        }
        
        return load;
    }
    
    vector<int> diffWaysToCompute(string input) {
        vector<int> res;
        int len = input.length();
        if(len == 0)  return res;
        if(isNumber(input))  {
            res.push_back(toInt(input));
            return res;
        }
        
        vector<int> l, r;
        for(int i=0; i<len; ++i) {
            if(input[i] == '-') {
                l = dfs(input.substr(0, i));
                r = dfs(input.substr(i+1, len-i-1));
                for(int p=0; p<l.size(); ++p) {
                    for(int q=0; q<r.size(); ++q) {
                        res.push_back(l[p] - r[q]);
                    }
                }
            }
            else if(input[i] == '+') {
                l = dfs(input.substr(0, i));
                r = dfs(input.substr(i+1, len-i-1));
                for(int p=0; p<l.size(); ++p) {
                    for(int q=0; q<r.size(); ++q) {
                        res.push_back(l[p] + r[q]);
                    }
                }
            }
            else if(input[i] == '*') {
                l = dfs(input.substr(0, i));
                r = dfs(input.substr(i+1, len-i-1));
                for(int p=0; p<l.size(); ++p) {
                    for(int q=0; q<r.size(); ++q) {
                        res.push_back(l[p] * r[q]);
                    }
                }
            }            
        }
        
        //sort(res.begin(), res.end());
        return res;
    }
};