就是楼上说的两种。。。正推和反算即可。。。帖份代码给不想dance的同学们研究下吧。。。
这里说的对剪枝蛮有启发。。。http://www.sudokufans.org.cn/forums/topic/8/

#include<stdio.h>
#include<algorithm>
#include<string.h>
using namespace std;

struct node{
    int x;
    int y;
    int option;
    bool optionList[9];
};

int table[9][9];
bool row[9][9];
bool col[9][9];
bool block[9][9];
node order[81];

int cmp(const node& a, const node& b) {
    return a.option < b.option;
}

void init() {
    memset(row, 0, sizeof(row));
    memset(col, 0, sizeof(col));
    memset(block, 0, sizeof(block));
    for(int i = 0; i < 9; i++) {
        for(int j = 0; j < 9; j++) {
            if(table[i][j] != 0) {
                row[i][table[i][j]-1] = true;
                col[j][table[i][j]-1] = true;
                block[i/3*3 + j/3][table[i][j]-1] = true;
            }
        }
    }
}

void updateOption(int l) {
    int rowOption[9][9] = {};
    int colOption[9][9] = {};
    int blockOption[9][9] = {};
    for(int index = 0; index < l; index++) {
        int i = order[index].x;
        int j = order[index].y;
        if(table[i][j] == 0) {
            order[index].option = 0;
            memset(order[index].optionList, 0, sizeof(order[index].optionList));
            for(int k = 0; k < 9; k++) {
                if(!row[i][k] && !col[j][k] && !block[i/3*3 + j/3][k]) {
                    order[index].option++;
                    order[index].optionList[k] = true;
                    rowOption[i][k]++;
                    colOption[j][k]++;
                    blockOption[i/3*3 + j/3][k]++;
                }
            }
        }
    }
    for(int n = 0; n < 9; n++) {
        for(int k = 0; k < 9; k++) {
            if(rowOption[n][k] == 1) {
                for(int index = 0; index < l; index++) {
                    if(table[order[index].x][order[index].y] == 0) {
                        if(order[index].x == n && order[index].optionList[k]) {
                            order[index].option = 1;
                            memset(order[index].optionList, 0, sizeof(order[index].optionList));
                            order[index].optionList[k] = true;
                        }
                    }
                }
            }
            if(colOption[n][k] == 1) {
                for(int index = 0; index < l; index++) {
                    if(table[order[index].x][order[index].y] == 0) {
                        if(order[index].y == n && order[index].optionList[k]) {
                            order[index].option = 1;
                            memset(order[index].optionList, 0, sizeof(order[index].optionList));
                            order[index].optionList[k] = true;
                        }
                    }
                }
            }
           if(blockOption[n][k] == 1) {
                for(int index = 0; index < l; index++) {
                    if(table[order[index].x][order[index].y] == 0) {
                        if(order[index].x/3*3 + order[index].y/3 == n && order[index].optionList[k]) {
                            order[index].option = 1;
                            memset(order[index].optionList, 0, sizeof(order[index].optionList));
                            order[index].optionList[k] = true;
                        }
                    }
                }               
            }
        }
    }
}

int getOrder() {
    memset(order, 0, sizeof(order));
    int index = 0;
    for(int i = 0; i < 9; i++) {
        for(int j = 0; j < 9; j++) {
            if(table[i][j] == 0) {
                order[index].x = i;
                order[index].y = j;
                index++;
            }
        }
    }
    updateOption(index);
    return index;
}

void inputOne(int i, int j, int k) {
    row[i][k] = true;
    col[j][k] = true;
    block[i/3*3+j/3][k] = true;
    table[i][j] = k+1;
}

void outputOne(int i, int j, int k) {
    row[i][k] = false;
    col[j][k] = false;
    block[i/3*3+j/3][k] = false;
    table[i][j] = 0;
}

int getIndex(int l) {
    int option = 10;
    int res = -1;
    for(int index = 0; index < l; index++) {
        int i = order[index].x;
        int j = order[index].y;
        if(table[i][j] == 0) {
            if(order[index].option < option) {
                option = order[index].option;
                res = index;
            }
        }
    }
    if(option == 0) {
        return -1;
    }
    else {
        return res;
    }
}

bool dfs(int cnt, int l) {
    if(cnt == l)
        return true;
    int index = getIndex(l);
    if(index == -1)
        return false;
    int i = order[index].x;
    int j = order[index].y;
    for(int k = 0; k < 9; k++) {
        if(order[index].optionList[k]) {
            inputOne(i, j, k);
            updateOption(l);
            if(dfs(cnt + 1, l)) {
                return true;
            }
            outputOne(i, j, k);
            updateOption(l);
       }
    }
    return false;
}

int main() {
    char tmp[82] = {};
    while(scanf("%s", tmp) && strcmp(tmp, "end") != 0) {
        for(int i = 0; i < 9; i++) {
            for(int j = 0; j < 9; j++) {
                if(tmp[i*9 + j] == '.') {
                    table[i][j] = 0;
                }
                else {
                    table[i][j] = tmp[i*9 + j] - '0';
                }
            }
        }
        init();
        int l = getOrder();

        dfs(0, l);
        for(int i = 0; i < 9; i++) {
            for(int j = 0; j < 9; j++) {
                printf("%d", table[i][j]);
            }
        }
        printf("\n");
    }
}