day27 贪心1

贪心解题步骤：

1.将问题分解为若干子问题
2.找出适合的贪心策略
3.求解每个子问题的最优解
4.将局部最优解堆叠成全局最优解

455.分发饼干

class Solution {
public:
    int findContentChildren(vector<int>& g, vector<int>& s) {
        int res = 0;
        sort(g.begin(), g.end());
        sort(s.begin(), s.end());
        for (int i = 0, j = 0; i < g.size() && j < s.size(); j ++) {
            if (s[j] >= g[i]) {
                res ++;
                i ++;
            }
        }
        return res;
    }
};

376. 摆动序列

贪心法：

class Solution {
public:
    int wiggleMaxLength(vector<int>& nums) {
        if (nums.size() <= 1) {
            return nums.size();
        }
        int curDiff = 0;
        int preDiff = 0;
        int res = 1;
        //以i结束
        for (int i = 0; i < nums.size() - 1; i ++) {
            curDiff = nums[i+1] - nums[i];
            if ((preDiff <= 0 && curDiff > 0) || (preDiff >= 0 && curDiff < 0)) {
                res ++;
                preDiff = curDiff;
            }
        }
        return res;
    }
};

53. 最大子序和

class Solution {
public:
    int maxSubArray(vector<int>& nums) {
        vector<int> dp(nums.size() + 1, 0);
        int res = nums[0];
        for (int i = 0; i < nums.size(); i ++) {
            if (dp[i] > 0) {
                dp[i+1] = dp[i] + nums[i];
            }
            else {
                dp[i+1] = nums[i];
            }
            res = max(res, dp[i+1]);
        }
        return res;
    }
};