[LeetCode]First Missing Positive

标签：leetcode

    public int firstMissingPositive1(int[] A) {// big O(N*N)
        if(A.length == 0)return 1;

        int i = 1;
        while(i <= A.length){
            boolean isfind = false;
            for(int j = 0; j< A.length; ++j){
                if(A[j] == i){
                    isfind = true;
                    break;
                }
            }
            if(!isfind) break;
            i ++;
        }
        return i;
    }

    public int firstMissingPositive2(int[] A) {//sort First O(N*log(N))+O(N)
        if(A.length == 0)return 1;

        Arrays.sort(A);
        int currNotFind = 1, i = 0;
        while(i <= A.length){
            if(currNotFind == A[i]){
                currNotFind  ++ ;
                i++;
            }else if(currNotFind  < A[i]){
                return currNotFind ;
            }else {
                i ++;
            }
        }
        return currNotFind;
    }

    public int firstMissingPositive3(int[] A) {//O(N) and O(N)space
        if(A.length == 0)return 1;

        int [] find = new int[A.length + 1];

        for (int i =0; i < A.length; ++i){
            if(A[i] > 0 && A[i] <= A.length){
                find[A[i]-1] = 1;
            }
        }
        for (int i = 0; i <= A.length; ++i){
            if(find[i] == 0)return i+1;
        }
        return 1;
    }

思路4：不开劈额外空间，但是这需要在元数组中进行置换，将元素i放到数组中的i-1的位置上去，空间复杂度O（1），时间复杂度O（N）
代码：

    public int firstMissingPositive(int[] A) {//O(N) and O(1)
        if(A.length == 0)return 1;
        int temp =0;

        for (int i =0; i < A.length;){
            if(A[i] != (i+1) && A[i] >= 1 && A[i] <= A.length && A[A[i]-1] != A[i]){
                temp = A[i];
                A[i] = A[temp - 1];
                A[temp-1] = temp;
            }else ++i;
        }
        int i = 0;
        for (i = 0; i < A.length; ++i){
            if(A[i] != i+1)return i+1;
        }
        return A.length + 1;
    }

[LeetCode]First Missing Positive

标签：leetcode

原文地址：http://blog.csdn.net/youmengjiuzhuiba/article/details/44891629