strStr
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strstr (a.k.a find sub string), is a useful function in string operation. Your task is to implement this function.
For a given source string and a target string, you should output the firstindex(from 0) of target string in source string.
If target does not exist in source, just return -1
.
If source = "source"
and target = "target"
, return -1
.
If source = "abcdabcdefg"
and target = "bcd"
, return 1
.
O(n2) is acceptable. Can you implement an O(n) algorithm? (hint: KMP)
Do I need to implement KMP Algorithm in a real interview?
- Not necessary. When you meet this problem in a real interview, the interviewer may just want to test your basic implementation ability. But make sure your confirm with the interviewer first.
题目至少有3种方法,第一种是暴力搜索,每次移动一步,则时间复杂度为O(nm),其中n表示source的长度,m表示target的长度。第二种方法是在第一种方法的基础上做了改进,每次对比时,同时从两边向中间夹,这样可以适当降低时间复杂度;第三种方法,肯定就是KMP了,KMP算法优化的是第一二种方法存在的回溯问题,第一二种方法中,每次移动i到达某个值,使得source[i] != target[j]的时候,会回溯到i - j + 1的位置,也就是说[i-j+1, i]这个区间又要重复计算;
KMP算法的提出就是要避免此回溯,使得i只往后移,也就是说,这样的时间复杂度将是O(N),N为source的长度。KMP的算法证明,就不详细展开了;这里主要讲下next[j]的求解,其实在求解next[j]的过程有递归的含义在其中,也是在做一个字符串的匹配问题,有两个变量k,j,分别初始化为k = -1, j = 0,next[0] = -1;当k == -1 或者 target[k] == target[j]的时候,则说明next[j + 1] = k + 1; 否则的话,则将k移动到next[k],这一步就包括了递推的字符串匹配在其中,理解起来比较难;
class Solution {public: /** * Returns a index to the first occurrence of target in source, * or -1 if target is not part of source. * @param source string to be scanned. * @param target string containing the sequence of characters to match. */ //KMP算法 void GetNext(const char *p, vector<int> &next) { int len = strlen(p); int k = -1, j = 0; while(j < len - 1) { if(k == -1 || p[k] == p[j]) { next[++j] = ++k; } else k = next[k]; } } int strStr(const char *source, const char *target) { // write your code here if(source == NULL || target == NULL) return -1; int lens = strlen(source); int lent = strlen(target); if(lens < lent) return -1; int i = 0, j = 0; vector<int> next(lent, -1); GetNext(target, next); while(i < lens && j < lent) { if(j == -1) { ++i; j = 0; } else if(source[i] == target[j]) { ++i; ++j; } else { j = next[j]; } } if(j >= lent) return i - j; return -1; }};
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