字符串全排列（JS版本）

题目描述

permutations('a'); // ['a']
permutations('ab'); // ['ab', 'ba']
permutations('aabb'); // ['aabb', 'abab', 'abba', 'baab', 'baba', 'bbaa']

题目思路

• 都是采用插空的方法。所以基本思路也就这样了，每次选一个字符，在剩余字符串中进行插空
• 递归

例子permutations('abc')

const permutations = string => {
    const result = [];
    if (string.length === 1) {
        return [string];
    } else {
        var pre = permutations(string.slice(1));
        for (var j = 0; j < pre.length; j++) {
            for (var k = 0; k < pre[j].length + 1; k++) {
                var temp = pre[j].slice(0, k) + string[0] + pre[j].slice(k);
                result.push(temp);
            }
        }
        return [...new Set(result)];
    }
}

代码分析

1. 当string = 'c'时，退出最后一次循环
2. 核心代码

var preResult = permutations(string.slice(1));
for (var j = 0; j < preResult.length; j++) {
    for (var k = 0; k < preResult[j].length+1; k++) {
            var temp = preResult[j].slice(0,k)+string[0]+preResult[j].slice(k);
            result.push(temp);         
        }
    }
return [...new Set(result)];

3. 当preResult = ['c'], string = "bc"

当 j = 0
preResult[j] = 'c'
k = 0
preResult[j].slice(0,k) = ''    string[0] = 'b'     preResult[j].slice(k) = 'c'
temp = 'bc'

k = 1
preResult[j].slice(0,k) = 'c'    string[0] = 'b'      preResult[j].slice(k) = ''
temp = 'cb'

4. 当preResult = ['bc', 'cb'], string = "abc"，string[0] = 'a'就是那个插入的值

当 j = 0
preResult[j] = 'bc'

k = 0
preResult[j].slice(0,k) = ''      string[0] = 'a'    preResult[j].slice(k) = 'bc'
temp = 'abc'

k = 1
preResult[j].slice(0,k) = 'b'      string[0] = 'a'    preResult[j].slice(k) = 'c'
temp = 'bac'

k = 2
preResult[j].slice(0,k) = 'bc'      string[0] = 'a'    preResult[j].slice(k) = ''
temp = 'bca'

当 j = 1
preResult[j] = 'cb'

k = 0
preResult[j].slice(0,k) = ''      string[0] = 'a'    preResult[j].slice(k) = 'cb'
temp = 'acb'

k = 1
preResult[j].slice(0,k) = 'c'      string[0] = 'a'    preResult[j].slice(k) = 'b'
temp = 'cab'

k = 2
preResult[j].slice(0,k) = 'cb'      string[0] = 'a'    preResult[j].slice(k) = ''
temp = 'cba'

总结

1. 当preResult = ['c'], string = "bc"时

即向'_c_'中插入'b',得到['bc', 'cb']

2. 当preResult = ['bc', 'cb'], string = "abc"，string[0] = 'a'就是那个插入的值

'_b_c_',_c_b_中插入'a',得到['abc', 'bac', 'bca', 'acb', 'cab', 'cba']

3. slice(0,k)和 slice(k)把字符串从k下标切割成两部分,中间插入需要的值