三、算法分类经典题目练习：贪心算法
贪心法：不断贪心的的选取当前最优策略，使用贪心算法的前提条件是当前最优解即是全局最优解。
455.分发饼干
class Solution {
public:
int findContentChildren(vector<int>& g, vector<int>& s) {
sort(g.begin(), g.end());
sort(s.begin(), s.end());
int child = 0, cookies = 0;
while(child < g.size() && cookies < s.size()){
if(g[child] <= s[cookies]) child++;
cookies++;
}
return child;
}
};
376.摆动序列
class Solution {
public:
int wiggleMaxLength(vector<int>& nums) {
if(nums.size()<2) return nums.size();
int max_len = 1;
static const int BEGIN = 0;
static const int UP = 1;
static const int DOWN = 2;
int state = BEGIN;
for(int i = 1; i < nums.size(); ++i){
switch(state){
case BEGIN:
if(nums[i-1] < nums[i]){
state = UP;
max_len++;
}else if(nums[i-1] > nums[i]){
state = DOWN;
max_len++;
}
break;
case UP:
if(nums[i-1] > nums[i]){
state = DOWN;
max_len++;
}
break;
case DOWN:
if(nums[i-1] < nums[i]){
state = UP;
max_len++;
}
break;
}
}
return max_len;
}
};
402.移掉K位数字
class Solution {
public:
string removeKdigits(string num, int k) {
vector<int> S;
string res = "";
for(int i = 0; i < num.size(); ++i){
int number = num[i]-'0';
while(S.size()!=0 && k>0 && S[S.size()-1]>number){
k--;
S.pop_back();
}
if(number!=0 || S.size()!=0) S.push_back(number);
}
while(S.size()!=0 && k>0) S.pop_back(),k--;
for(int i = 0; i < S.size(); ++i) res.append(1,'0'+S[i]);
if(res=="") res = "0";
return res;
}
};
55.跳跃游戏
class Solution {
public:
bool canJump(vector<int>& nums) {
vector<int> index;
for(int i = 0; i < nums.size(); ++i){
index.push_back(i+nums[i]);
}
int jump = 0;
int max_index = index[0];
while(jump<nums.size() && jump<=max_index){
if(max_index < index[jump]) max_index = index[jump];
jump++;
}
if(jump == nums.size()) return true;
return false;
}
};
45.跳跃游戏II
class Solution {
public:
int jump(vector<int>& nums) {
if(nums.size()<2) return 0;
int jump_min = 1;
int cur_max_idx = nums[0];
int pre_max_idx = nums[0];
for(int i = 0; i < nums.size(); ++i){
if(i > cur_max_idx){
jump_min++;
cur_max_idx = pre_max_idx;
}
if(pre_max_idx < nums[i]+i){
pre_max_idx = nums[i]+i;
}
}
return jump_min;
}
};
452.用最少数量的箭引爆气球
class Solution {
public:
int findMinArrowShots(vector<vector<int>>& points) {
if(points.size()==0) return 0;
sort(points.begin(), points.end());
int shoot_num = 1;
int begin = points[0][0];
int end = points[0][1];
for(int i = 1; i < points.size(); ++i){
if(points[i][0] <= end){
begin = points[i][0];
if(points[i][1] < end){
end = points[i][1];
}
}else{
shoot_num++;
begin = points[i][0];
end = points[i][1];
}
}
return shoot_num;
}
};
poj 2431 Expedition
http://poj.org/problem?id=2431
#include