[Leetcode] Permutations

Permutations 题解

题目来源：https://leetcode.com/problems/permutations/description/

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Description

Given a collection of distinct numbers, return all possible permutations.

Example

For example,

[1,2,3] have the following permutations:

[
  [1,2,3],
  [1,3,2],
  [2,1,3],
  [2,3,1],
  [3,1,2],
  [3,2,1]
]

Solution

class Solution {
private:
    void backTrack(vector<vector<int> >& res, vector<int>& nums, int index) {
        if (index >= nums.size()) {
            res.push_back(nums);
        } else {
            int i, size = nums.size();
            for (i = index; i < size; i++) {
                swap(nums[i], nums[index]);
                backTrack(res, nums, index + 1);
                swap(nums[i], nums[index]);
            }
        }
    }
public:
    vector<vector<int> > permute(vector<int>& nums) {
        vector<vector<int> > res;
        if (nums.empty())
            return res;
        backTrack(res, nums, 0);
        return res;
    }
};

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解题描述

这道题题意是求给定数组（元素互不相同）的全排列。用到的算法是递归回溯，元素交换起始位index初值为0，每次进入回溯函数交换index及其后某个位置上的元素，再将index + 1使用递归回溯，以穷尽所有可能的排列情况并且不会重复相同情形。