递归和回溯法

17. Letter Combinations of a Phone Number

vector<string> letterCombinations(string digits) {
    vector<string> result;
    if(digits.empty()) return vector<string>();
    static const vector<string> v = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
    result.push_back("");   // add a seed for the initial case
    for(int i = 0 ; i < digits.size(); ++i) {
        int num = digits[i]-'0';
        if(num < 0 || num > 9) break;
        const string& candidate = v[num];
        if(candidate.empty()) continue;
        vector<string> tmp;
        for(int j = 0 ; j < candidate.size() ; ++j) {
            for(int k = 0 ; k < result.size() ; ++k) {
                tmp.push_back(result[k] + candidate[j]);
            }
        }
        result.swap(tmp);
    }
    return result;
}

---

93. Restore IP Addresses

---

131. Palindrome Partitioning

---

排列组合

46. Permutations

class Solution {
public:
    vector<vector<int> > permute(vector<int> &num) {
        vector<vector<int> > result;

        permuteRecursive(num, 0, result);
        return result;
    }

    // permute num[begin..end]
    // invariant: num[0..begin-1] have been fixed/permuted
    void permuteRecursive(vector<int> &num, int begin, vector<vector<int> > &result)    {
        if (begin >= num.size()) {
            // one permutation instance
            result.push_back(num);
            return;
        }

        for (int i = begin; i < num.size(); i++) {
            swap(num[begin], num[i]);
            permuteRecursive(num, begin + 1, result);
            // reset
            swap(num[begin], num[i]);
        }
    }
};

---

47. Permutations II

---

组合问题

77. Combinations

剪枝思想

39. Combination Sum

---

40. Combination Sum II

---

216. Combination Sum III

---

78. Subsets

---

90. Subsets II

---

401. Binary Watch

---

二维平面上使用回溯法

---

79. Word Search

floodfill算法，深度优先遍历

200. Number of Islands

---

130. Surrounded Regions

---

417. Pacific Atlantic Water Flow

---

N皇后问题

51. N-Queens

---

52. N-Queens II

---

37. Sudoku Solver