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+

第二轮：

卧槽，代码写的太烂简直不忍直视

class Solution {
public:
    /**
     * @param A: A list of lists of integers
     * @return: An integer
     */
    int jump(vector<int> A) {
        // wirte your code here
        int count = 0;
        int len = A.size();
        int sofar = 0;
        
        while (sofar < len) {
            count++;
            int end = min(sofar + A[sofar] + 1, len);
            if (end >= len) {
                break;
            }
            int maxrange = 0;
            for (int i = sofar; i < end; i++) {
                if (A[i] + i > maxrange) {
                    maxrange = A[i] + i;
                    sofar = i;
                }
            }
        }
        
        return count;
    }
};

更简单的方法：

class Solution {
public:
    /**
     * @param A: A list of lists of integers
     * @return: An integer
     */
    int jump(vector<int> A) {
        // wirte your code here
        int count = 0;
        int len = A.size();
        int sofar = 0;
        int jpfar = 0;
        for (int i = 0; i < len; i++) {
            if (jpfar < i) {
                count++;
                jpfar = sofar;
            }
            sofar = max(sofar, A[i] + i);
        }
        
        return count;
    }
};

参见：http://www.cnblogs.com/ganganloveu/p/3761715.html