LeetCode Jump Game II

class Solution {

private:

    vector<long> sum;

    vector<long> step;

    set<int> can;

public:

    int jump(int A[], int n) {

        step.clear(), sum.clear(), can.clear();

        step.resize(n, INT_MAX), sum.resize(n, 0);

        

        int jmp = A[0];

        can.insert(n - 1);

        step[n - 1] = 0;

        

        for (int i = n - 2; i>= 0; i--) {

            jmp = A[i];

            if (i + jmp >= n - 1) {

                sum[i] = sum[i + 1] + 1;

                can.insert(i);

                continue;

            }

            int region = sum[i + 1] - sum[i + jmp + 1];

            if (region > 0) {

                sum[i] = sum[i + 1] + 1;

                can.insert(i);

            } else {

                sum[i] = sum[i + 1];

            }

        }



        if (can.size() > 1 && (jmp >= n-1 || sum[0] - sum[jmp] > 0)) {

            can.insert(0);

        }



        int cur_jmp_to;

        int pre_jmp_to = n-1 + A[n - 1];



        for (int i = n - 2; i >= 0; i--) {

            cur_jmp_to = i + A[i];

            int min_step = INT_MAX;



            if (cur_jmp_to >= pre_jmp_to) {

                min_step = step[i + 1] - 1;

                if (min_step < 0) min_step = 0;

                for (int j = pre_jmp_to; j < n && j <= cur_jmp_to; j++) {

                    if (step[j] < min_step) min_step = step[j];

                }

            } else if (A[i] > 0){

                set<int>::iterator lower = can.lower_bound(i+1);

                set<int>::iterator upper = can.upper_bound(cur_jmp_to);

                while (lower != upper) {

                    if (step[*lower] < min_step) min_step = step[*lower];

                    lower++;

                }

            }

            step[i] = min_step == INT_MAX ? INT_MAX : min_step + 1;

            pre_jmp_to = cur_jmp_to;

        }

        

        return step[0];

    }

};

状态不佳，错了好多，100ms+，感觉好吃力。想起以前高中做数学题那会儿，看叼的人两三下把题目做出来了，自己写满一页还没算出来，真是tmd的不是滋味，不找规律，强来就是这样的结果，我tmd已经看穿了。

时间O（n），空间O（1）算法参考：http://www.cnblogs.com/zhuli19901106/p/3568216.html

// cloned from zhuli

class Solution {

public:

    int jump(int A[], int n) {

        if (A == nullptr || n <= 0) {

            return -1;

        } else if (n == 1) {

            return 0;

        }

        

        int last_pos, this_pos;

        int i;

        int result;

        

        last_pos = 0;

        this_pos = 0;

        result = 0;

        for (i = 0; this_pos < n - 1; ++i) {

            if (i > this_pos) {

                return -1;

            }

            if (i + A[i] > this_pos) {

                if (i > last_pos) {

                    last_pos = this_pos;

                    ++result;

                }

                this_pos = i + A[i];

            }

        }

        

        return result + 1;

    }

};

时间上快了一倍50ms+