Minimum Window Substring

Description:

Given a string source and a string target, find the minimum window in source which will contain all the characters in target.

Example

source = "ADOBECODEBANC" target = "ABC" Minimum window is "BANC".

Challenge

Can you do it in time complexity O(n) ?

Clarification

Should the characters in minimum window has the same order in target?

- Not necessary.

Solution:

class Solution {
public:    
	/**
	 * @param source: A string
	 * @param target: A string
	 * @return: A string denote the minimum window
	 *          Return "" if there is no such a string
	 */
	string minWindow(string &source, string &target) {
		auto l1 = (int)source.length();
		auto l2 = (int)target.length();
		if (l2 == 0 || l1 < l2) return "";
		int require[256] = {0};
		bool exists[256] = {false};
		int count = l2;
		for (int i = 0; i < l2; ++i) {
			++require[target[i]];
			exists[target[i]] = true;
		}
		int minLen = INT_MAX, minL = -1;
		int left = 0, right = 0;
		while (right <= l1) {
			if (count > 0) {
				if (right == l1) break;
				if (require[source[right]] > 0) --count;
				if (exists[source[right]]) --require[source[right]];
				++right;
			} else {
				if (right-left < minLen) {
					minLen = right-left;
					minL = left;
				}
				if (exists[source[left]]) ++require[source[left]];
				if (require[source[left]] > 0) ++count;
				++left;
			}
		}
		return minLen > l1 ? "" : source.substr(minL, minLen);
	}
};